Java Decision Making
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There are two types of decision making statements in Java. They are:
if statements
switch statements
The if Statement:
An if statement consists of a Boolean expression followed by one or more statements.
Syntax:
The syntax of an if statement is:
if(Boolean_expression)
{
//Statements will execute if the Boolean expression is true
}
If the Boolean expression evaluates to true then the block of code inside the if statement will be executed. If not the first set of code after the end of the if statement (after the closing curly brace) will be executed.
Example:
public class Test {
public static void main(String args[]){
int x = 10;
if( x < 20 ){
System.out.print("This is if statement");
}
}
}
This would produce the following result:
This is if statement
The if...else Statement:
An if statement can be followed by an optional else statement, which executes when the Boolean expression is false.
Syntax:
The syntax of an if...else is:
if(Boolean_expression){
//Executes when the Boolean expression is true
}else{
//Executes when the Boolean expression is false
}
Example:
public class Test {
public static void main(String args[]){
int x = 30;
if( x < 20 ){
System.out.print("This is if statement");
}else{
System.out.print("This is else statement");
}
}
}
This would produce the following result:
This is else statement
The if...else if...else Statement:
An if statement can be followed by an optional else if...else statement, which is very useful to test various conditions using single if...else if statement.
When using if , else if , else statements there are few points to keep in mind.
An if can have zero or one else's and it must come after any else if's.
An if can have zero to many else if's and they must come before the else.
Once an else if succeeds, none of the remaining else if's or else's will be tested.
Syntax:
The syntax of an if...else is:
if(Boolean_expression 1){
//Executes when the Boolean expression 1 is true
}else if(Boolean_expression 2){
//Executes when the Boolean expression 2 is true
}else if(Boolean_expression 3){
//Executes when the Boolean expression 3 is true
}else {
//Executes when the none of the above condition is true.
}
Example:
public class Test {
public static void main(String args[]){
int x = 30;
if( x == 10 ){
System.out.print("Value of X is 10");
}else if( x == 20 ){
System.out.print("Value of X is 20");
}else if( x == 30 ){
System.out.print("Value of X is 30");
}else{
System.out.print("This is else statement");
}
}
}
This would produce the following result:
Value of X is 30
Nested if...else Statement:
It is always legal to nest if-else statements which means you can use one if or else if statement inside another if or else if statement.
Syntax:
The syntax for a nested if...else is as follows:
if(Boolean_expression 1){
//Executes when the Boolean expression 1 is true
if(Boolean_expression 2){
//Executes when the Boolean expression 2 is true
}
}
You can nest else if...else in the similar way as we have nested if statement.
Example:
Public class Test {
public static void main(String args[]){
int x = 30;
int y = 10;
if( x == 30 ){
if( y == 10 ){
System.out.print("X = 30 and Y = 10");
}
}
}
}
This would produce the following result:
X = 30 and Y = 10
The switch Statement:
A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each case.
Syntax:
The syntax of enhanced for loop is:
switch(expression){
case value :
//Statements
break; //optional
case value :
//Statements
break; //optional
//You can have any number of case statements.
default : //Optional
//Statements
}
The following rules apply to a switch statement:
The variable used in a switch statement can only be a byte, short, int, or char.
You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon.
The value for a case must be the same data type as the variable in the switch and it must be a constant or a literal.
When the variable being switched on is equal to a case, the statements following that case will execute until a break statement is reached.
When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.
Not every case needs to contain a break. If no break appears, the flow of control will fall through to subsequent cases until a break is reached.
A switch statement can have an optional default case, which must appear at the end of the switch. The default case can be used for performing a task when none of the cases is true. No break is needed in the default case.
Example:
public class Test {
public static void main(String args[]){
//char grade = args[0].charAt(0);
char grade = 'C';
switch(grade)
{
case 'A' :
System.out.println("Excellent!");
break;
case 'B' :
case 'C' :
System.out.println("Well done");
break;
case 'D' :
System.out.println("You passed");
case 'F' :
System.out.println("Better try again");
break;
default :
System.out.println("Invalid grade");
}
System.out.println("Your grade is " + grade);
}
}
Compile and run above program using various command line arguments. This would produce the following result:
$ java Test
Well done
Your grade is a C
$
What is Next?
Next chapter discuses about the Number class (in the java.lang package) and its subclasses in Java Language.
We will be looking into some of the situations where you would use instantiations of these classes rather than the primitive data types, as well as classes such as formatting, mathematical functions that you need to know about when working with Numbers.
Tuesday, 31 December 2013
Java Loops - for, while and do...while
Java Loops - for, while and do...while
Previous Page Next Page
There may be a situation when we need to execute a block of code several number of times, and is often referred to as a loop.
Java has very flexible three looping mechanisms. You can use one of the following three loops:
while Loop
do...while Loop
for Loop
As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.
The while Loop:
A while loop is a control structure that allows you to repeat a task a certain number of times.
Syntax:
The syntax of a while loop is:
while(Boolean_expression)
{
//Statements
}
When executing, if the boolean_expression result is true, then the actions inside the loop will be executed. This will continue as long as the expression result is true.
Here, key point of the while loop is that the loop might not ever run. When the expression is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.
Example:
public class Test {
public static void main(String args[]) {
int x = 10;
while( x < 20 ) {
System.out.print("value of x : " + x );
x++;
System.out.print("\n");
}
}
}
This would produce the following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
The do...while Loop:
A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.
Syntax:
The syntax of a do...while loop is:
do
{
//Statements
}while(Boolean_expression);
Notice that the Boolean expression appears at the end of the loop, so the statements in the loop execute once before the Boolean is tested.
If the Boolean expression is true, the flow of control jumps back up to do, and the statements in the loop execute again. This process repeats until the Boolean expression is false.
Example:
public class Test {
public static void main(String args[]){
int x = 10;
do{
System.out.print("value of x : " + x );
x++;
System.out.print("\n");
}while( x < 20 );
}
}
This would produce the following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
The for Loop:
A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.
A for loop is useful when you know how many times a task is to be repeated.
Syntax:
The syntax of a for loop is:
for(initialization; Boolean_expression; update)
{
//Stateme is the flow of control in a for loop:
The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.
Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement past the for loop.
After the body of the for loop executes, the flow of control jumps back up to the update statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the Boolean expression.
The Boolean expression is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then update step, then Boolean expression). After the Boolean expression is false, the for loop terminates.
Example:
public class Test {
public static void main(String args[]) {
for(int x = 10; x < 20; x = x+1) {
System.out.print("value of x : " + x );
System.out.print("\n");
}
}
}
This would produce the following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value value of x : 17
value of x : 18
value of x : 19
Enhanced for loop in Java:
As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.
Syntax:
The syntax of enhanced for loop is:
for(declaration : expression)
{
//Statements
}
Declaration: The newly declared block variable, which is of a type compatible with the elements of the array you are accessing. The variable will be available within the for block and its value would be the same as the current array element.
Expression: This evaluates to the array you need to loop through. The expression can be an array variable or method call that returns an array.
Example:
public class Test {
public static void main(String args[]){
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ){
System.out.print( x );
System.out.print(",");
}
System.out.print("\n");
String [] names ={"James", "Larry", "Tom", "Lacy"};
for( String name : names ) {
System.out.print( name );
System.out.print(",");
}
}
}
This would produce the following result:
10,20,30,40,50,
James,Larry,Tom,Lacy,
The break Keyword:
The break keyword is used to stop the entire loop. The break keyword must be used inside any loop or a switch statement.
The break keyword will stop the execution of the innermost loop and start executing the next line of code after the block.
Syntax:
The syntax of a break is a single statement inside any loop:
break;
Example:
public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
break;
}
System.out.print( x );
System.out.print("\n");
}
}
}
This would produce the following result:
10
20
The continue Keyword:
The continue keyword can be used in any of the loop control structures. It causes the loop to immediately jump to the next iteration of the loop.
In a for loop, the continue keyword causes flow of control to immediately jump to the update statement.
In a while loop or do/while loop, flow of control immediately jumps to the Boolean expression.
Syntax:
The syntax of a continue is a single statement inside any loop:
continue;
Example:
public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
continue;
}
System.out.print( x );
System.out.print("\n");
}
}
}
This would produce the following result:
10
20
40
50
What is Next?
In the following chapter, we will be learning about decision making statements in Java programming.
Previous Page Next Page
There may be a situation when we need to execute a block of code several number of times, and is often referred to as a loop.
Java has very flexible three looping mechanisms. You can use one of the following three loops:
while Loop
do...while Loop
for Loop
As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.
The while Loop:
A while loop is a control structure that allows you to repeat a task a certain number of times.
Syntax:
The syntax of a while loop is:
while(Boolean_expression)
{
//Statements
}
When executing, if the boolean_expression result is true, then the actions inside the loop will be executed. This will continue as long as the expression result is true.
Here, key point of the while loop is that the loop might not ever run. When the expression is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.
Example:
public class Test {
public static void main(String args[]) {
int x = 10;
while( x < 20 ) {
System.out.print("value of x : " + x );
x++;
System.out.print("\n");
}
}
}
This would produce the following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
The do...while Loop:
A do...while loop is similar to a while loop, except that a do...while loop is guaranteed to execute at least one time.
Syntax:
The syntax of a do...while loop is:
do
{
//Statements
}while(Boolean_expression);
Notice that the Boolean expression appears at the end of the loop, so the statements in the loop execute once before the Boolean is tested.
If the Boolean expression is true, the flow of control jumps back up to do, and the statements in the loop execute again. This process repeats until the Boolean expression is false.
Example:
public class Test {
public static void main(String args[]){
int x = 10;
do{
System.out.print("value of x : " + x );
x++;
System.out.print("\n");
}while( x < 20 );
}
}
This would produce the following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value of x : 17
value of x : 18
value of x : 19
The for Loop:
A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.
A for loop is useful when you know how many times a task is to be repeated.
Syntax:
The syntax of a for loop is:
for(initialization; Boolean_expression; update)
{
//Stateme is the flow of control in a for loop:
The initialization step is executed first, and only once. This step allows you to declare and initialize any loop control variables. You are not required to put a statement here, as long as a semicolon appears.
Next, the Boolean expression is evaluated. If it is true, the body of the loop is executed. If it is false, the body of the loop does not execute and flow of control jumps to the next statement past the for loop.
After the body of the for loop executes, the flow of control jumps back up to the update statement. This statement allows you to update any loop control variables. This statement can be left blank, as long as a semicolon appears after the Boolean expression.
The Boolean expression is now evaluated again. If it is true, the loop executes and the process repeats itself (body of loop, then update step, then Boolean expression). After the Boolean expression is false, the for loop terminates.
Example:
public class Test {
public static void main(String args[]) {
for(int x = 10; x < 20; x = x+1) {
System.out.print("value of x : " + x );
System.out.print("\n");
}
}
}
This would produce the following result:
value of x : 10
value of x : 11
value of x : 12
value of x : 13
value of x : 14
value of x : 15
value of x : 16
value value of x : 17
value of x : 18
value of x : 19
Enhanced for loop in Java:
As of Java 5, the enhanced for loop was introduced. This is mainly used for Arrays.
Syntax:
The syntax of enhanced for loop is:
for(declaration : expression)
{
//Statements
}
Declaration: The newly declared block variable, which is of a type compatible with the elements of the array you are accessing. The variable will be available within the for block and its value would be the same as the current array element.
Expression: This evaluates to the array you need to loop through. The expression can be an array variable or method call that returns an array.
Example:
public class Test {
public static void main(String args[]){
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ){
System.out.print( x );
System.out.print(",");
}
System.out.print("\n");
String [] names ={"James", "Larry", "Tom", "Lacy"};
for( String name : names ) {
System.out.print( name );
System.out.print(",");
}
}
}
This would produce the following result:
10,20,30,40,50,
James,Larry,Tom,Lacy,
The break Keyword:
The break keyword is used to stop the entire loop. The break keyword must be used inside any loop or a switch statement.
The break keyword will stop the execution of the innermost loop and start executing the next line of code after the block.
Syntax:
The syntax of a break is a single statement inside any loop:
break;
Example:
public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
break;
}
System.out.print( x );
System.out.print("\n");
}
}
}
This would produce the following result:
10
20
The continue Keyword:
The continue keyword can be used in any of the loop control structures. It causes the loop to immediately jump to the next iteration of the loop.
In a for loop, the continue keyword causes flow of control to immediately jump to the update statement.
In a while loop or do/while loop, flow of control immediately jumps to the Boolean expression.
Syntax:
The syntax of a continue is a single statement inside any loop:
continue;
Example:
public class Test {
public static void main(String args[]) {
int [] numbers = {10, 20, 30, 40, 50};
for(int x : numbers ) {
if( x == 30 ) {
continue;
}
System.out.print( x );
System.out.print("\n");
}
}
}
This would produce the following result:
10
20
40
50
What is Next?
In the following chapter, we will be learning about decision making statements in Java programming.
Java Basic Operators
Java Basic Operators
Java provides a rich set of operators to manipulate variables. We can divide all the Java operators into the following groups:
Arithmetic Operators
Relational Operators
Bitwise Operators
Logical Operators
Assignment Operators
Misc Operators
The Arithmetic Operators:
Arithmetic operators are used in mathematical expressions in the same way that they are used in algebra. The following table lists the arithmetic operators:
Assume integer variable A holds 10 and variable B holds 20, then:
Show Examples
Operator Description Example
+ Addition - Adds values on either side of the operator A + B will give 30
- Subtraction - Subtracts right hand operand from left hand operand A - B will give -10
* Multiplication - Multiplies values on either side of the operator A * B will give 200
/ Division - Divides left hand operand by right hand operand B / A will give 2
% Modulus - Divides left hand operand by right hand operand and returns remainder B % A will give 0
++ Increment - Increases the value of operand by 1 B++ gives 21
-- Decrement - Decreases the value of operand by 1 B-- gives 19
The Relational Operators:
There are following relational operators supported by Java language
Assume variable A holds 10 and variable B holds 20, then:
Show Examples
Operator Description Example
== Checks if the values of two operands are equal or not, if yes then condition becomes true. (A == B) is not true.
!= Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. (A != B) is true.
> Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. (A > B) is not true.
< Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (A < B) is true.
>= Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. (A >= B) is not true.
<= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. (A <= B) is true.
The Bitwise Operators:
Java defines several bitwise operators, which can be applied to the integer types, long, int, short, char, and byte.
Bitwise operator works on bits and performs bit-by-bit operation. Assume if a = 60; and b = 13; now in binary format they will be as follows:
a = 0011 1100
b = 0000 1101
-----------------
a&b = 0000 1100
a|b = 0011 1101
a^b = 0011 0001
~a = 1100 0011
The following table lists the bitwise operators:
Assume integer variable A holds 60 and variable B holds 13 then:
Show Examples
Operator Description Example
& Binary AND Operator copies a bit to the result if it exists in both operands. (A & B) will give 12 which is 0000 1100
| Binary OR Operator copies a bit if it exists in either operand. (A | B) will give 61 which is 0011 1101
^ Binary XOR Operator copies the bit if it is set in one operand but not both. (A ^ B) will give 49 which is 0011 0001
~ Binary Ones Complement Operator is unary and has the effect of 'flipping' bits. (~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number.
<< Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand. A << 2 will give 240 which is 1111 0000
>> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. A >> 2 will give 15 which is 1111
>>> Shift right zero fill operator. The left operands value is moved right by the number of bits specified by the right operand and shifted values are filled up with zeros. A >>>2 will give 15 which is 0000 1111
The Logical Operators:
The following table lists the logical operators:
Assume Boolean variables A holds true and variable B holds false, then:
Show Examples
Operator Description Example
&& Called Logical AND operator. If both the operands are non-zero, then the condition becomes true. (A && B) is false.
|| Called Logical OR Operator. If any of the two operands are non-zero, then the condition becomes true. (A || B) is true.
! Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false. !(A && B) is true.
The Assignment Operators:
There are following assignment operators supported by Java language:
Show Examples
Operator Description Example
= Simple assignment operator, Assigns values from right side operands to left side operand C = A + B will assign value of A + B into C
+= Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand C += A is equivalent to C = C + A
-= Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand C -= A is equivalent to C = C - A
*= Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand C *= A is equivalent to C = C * A
/= Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand C /= A is equivalent to C = C / A
%= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand C %= A is equivalent to C = C % A
<<= Left shift AND assignment operator C <<= 2 is same as C = C << 2
>>= Right shift AND assignment operator C >>= 2 is same as C = C >> 2
&= Bitwise AND assignment operator C &= 2 is same as C = C & 2
^= bitwise exclusive OR and assignment operator C ^= 2 is same as C = C ^ 2
|= bitwise inclusive OR and assignment operator C |= 2 is same as C = C | 2
Misc Operators
There are few other operators supported by Java Language.
Conditional Operator ( ? : ):
Conditional operator is also known as the ternary operator. This operator consists of three operands and is used to evaluate Boolean expressions. The goal of the operator is to decide which value should be assigned to the variable. The operator is written as:
variable x = (expression) ? value if true : value if false
Following is the example:
public class Test {
public static void main(String args[]){
int a , b;
a = 10;
b = (a == 1) ? 20: 30;
System.out.println( "Value of b is : " + b );
b = (a == 10) ? 20: 30;
System.out.println( "Value of b is : " + b );
}
}
This would produce the following result:
Value of b is : 30
Value of b is : 20
instanceOf Operator:
This operator is used only for object reference variables. The operator checks whether the object is of a particular type(class type or interface type). instanceOf operator is wriiten as:
( Object reference variable ) instanceOf (class/interface type)
If the object referred by the variable on the left side of the operator passes the IS-A check for the class/interface type on the right side, then the result will be true. Following is the example:
String name = = 'James';
boolean result = name instanceOf String;
// This will return true since name is type of String
This operator will still return true if the object being compared is the assignment compatible with the type on the right. Following is one more example:
class Vehicle {}
public class Car extends Vehicle {
public static void main(String args[]){
Vehicle a = new Car();
boolean result = a instanceof Car;
System.out.println( result);
}
}
This would produce the following result:
true
Precedence of Java Operators:
Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator:
For example, x = 7 + 3 * 2; here x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.
Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.
Category Operator Associativity
Postfix () [] . (dot operator) Left to right
Unary ++ - - ! ~ Right to left
Multiplicative * / % Left to right
Additive + - Left to right
Shift >> >>> << Left to right
Relational > >= < <= Left to right
Equality == != Left to right
Bitwise AND & Left to right
Bitwise XOR ^ Left to right
Bitwise OR | Left to right
Logical AND && Left to right
Logical OR || Left to right
Conditional ?: Right to left
Assignment = += -= *= /= %= >>= <<= &= ^= |= Right to left
Comma , Left to right
What is Next?
Next chapter would explain about loop control in Java programming. The chapter will describe various types of loops and how these loops can be used in Java program development and for what purposes they are being used
Java provides a rich set of operators to manipulate variables. We can divide all the Java operators into the following groups:
Arithmetic Operators
Relational Operators
Bitwise Operators
Logical Operators
Assignment Operators
Misc Operators
The Arithmetic Operators:
Arithmetic operators are used in mathematical expressions in the same way that they are used in algebra. The following table lists the arithmetic operators:
Assume integer variable A holds 10 and variable B holds 20, then:
Show Examples
Operator Description Example
+ Addition - Adds values on either side of the operator A + B will give 30
- Subtraction - Subtracts right hand operand from left hand operand A - B will give -10
* Multiplication - Multiplies values on either side of the operator A * B will give 200
/ Division - Divides left hand operand by right hand operand B / A will give 2
% Modulus - Divides left hand operand by right hand operand and returns remainder B % A will give 0
++ Increment - Increases the value of operand by 1 B++ gives 21
-- Decrement - Decreases the value of operand by 1 B-- gives 19
The Relational Operators:
There are following relational operators supported by Java language
Assume variable A holds 10 and variable B holds 20, then:
Show Examples
Operator Description Example
== Checks if the values of two operands are equal or not, if yes then condition becomes true. (A == B) is not true.
!= Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. (A != B) is true.
> Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. (A > B) is not true.
< Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. (A < B) is true.
>= Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. (A >= B) is not true.
<= Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. (A <= B) is true.
The Bitwise Operators:
Java defines several bitwise operators, which can be applied to the integer types, long, int, short, char, and byte.
Bitwise operator works on bits and performs bit-by-bit operation. Assume if a = 60; and b = 13; now in binary format they will be as follows:
a = 0011 1100
b = 0000 1101
-----------------
a&b = 0000 1100
a|b = 0011 1101
a^b = 0011 0001
~a = 1100 0011
The following table lists the bitwise operators:
Assume integer variable A holds 60 and variable B holds 13 then:
Show Examples
Operator Description Example
& Binary AND Operator copies a bit to the result if it exists in both operands. (A & B) will give 12 which is 0000 1100
| Binary OR Operator copies a bit if it exists in either operand. (A | B) will give 61 which is 0011 1101
^ Binary XOR Operator copies the bit if it is set in one operand but not both. (A ^ B) will give 49 which is 0011 0001
~ Binary Ones Complement Operator is unary and has the effect of 'flipping' bits. (~A ) will give -61 which is 1100 0011 in 2's complement form due to a signed binary number.
<< Binary Left Shift Operator. The left operands value is moved left by the number of bits specified by the right operand. A << 2 will give 240 which is 1111 0000
>> Binary Right Shift Operator. The left operands value is moved right by the number of bits specified by the right operand. A >> 2 will give 15 which is 1111
>>> Shift right zero fill operator. The left operands value is moved right by the number of bits specified by the right operand and shifted values are filled up with zeros. A >>>2 will give 15 which is 0000 1111
The Logical Operators:
The following table lists the logical operators:
Assume Boolean variables A holds true and variable B holds false, then:
Show Examples
Operator Description Example
&& Called Logical AND operator. If both the operands are non-zero, then the condition becomes true. (A && B) is false.
|| Called Logical OR Operator. If any of the two operands are non-zero, then the condition becomes true. (A || B) is true.
! Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false. !(A && B) is true.
The Assignment Operators:
There are following assignment operators supported by Java language:
Show Examples
Operator Description Example
= Simple assignment operator, Assigns values from right side operands to left side operand C = A + B will assign value of A + B into C
+= Add AND assignment operator, It adds right operand to the left operand and assign the result to left operand C += A is equivalent to C = C + A
-= Subtract AND assignment operator, It subtracts right operand from the left operand and assign the result to left operand C -= A is equivalent to C = C - A
*= Multiply AND assignment operator, It multiplies right operand with the left operand and assign the result to left operand C *= A is equivalent to C = C * A
/= Divide AND assignment operator, It divides left operand with the right operand and assign the result to left operand C /= A is equivalent to C = C / A
%= Modulus AND assignment operator, It takes modulus using two operands and assign the result to left operand C %= A is equivalent to C = C % A
<<= Left shift AND assignment operator C <<= 2 is same as C = C << 2
>>= Right shift AND assignment operator C >>= 2 is same as C = C >> 2
&= Bitwise AND assignment operator C &= 2 is same as C = C & 2
^= bitwise exclusive OR and assignment operator C ^= 2 is same as C = C ^ 2
|= bitwise inclusive OR and assignment operator C |= 2 is same as C = C | 2
Misc Operators
There are few other operators supported by Java Language.
Conditional Operator ( ? : ):
Conditional operator is also known as the ternary operator. This operator consists of three operands and is used to evaluate Boolean expressions. The goal of the operator is to decide which value should be assigned to the variable. The operator is written as:
variable x = (expression) ? value if true : value if false
Following is the example:
public class Test {
public static void main(String args[]){
int a , b;
a = 10;
b = (a == 1) ? 20: 30;
System.out.println( "Value of b is : " + b );
b = (a == 10) ? 20: 30;
System.out.println( "Value of b is : " + b );
}
}
This would produce the following result:
Value of b is : 30
Value of b is : 20
instanceOf Operator:
This operator is used only for object reference variables. The operator checks whether the object is of a particular type(class type or interface type). instanceOf operator is wriiten as:
( Object reference variable ) instanceOf (class/interface type)
If the object referred by the variable on the left side of the operator passes the IS-A check for the class/interface type on the right side, then the result will be true. Following is the example:
String name = = 'James';
boolean result = name instanceOf String;
// This will return true since name is type of String
This operator will still return true if the object being compared is the assignment compatible with the type on the right. Following is one more example:
class Vehicle {}
public class Car extends Vehicle {
public static void main(String args[]){
Vehicle a = new Car();
boolean result = a instanceof Car;
System.out.println( result);
}
}
This would produce the following result:
true
Precedence of Java Operators:
Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator:
For example, x = 7 + 3 * 2; here x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.
Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.
Category Operator Associativity
Postfix () [] . (dot operator) Left to right
Unary ++ - - ! ~ Right to left
Multiplicative * / % Left to right
Additive + - Left to right
Shift >> >>> << Left to right
Relational > >= < <= Left to right
Equality == != Left to right
Bitwise AND & Left to right
Bitwise XOR ^ Left to right
Bitwise OR | Left to right
Logical AND && Left to right
Logical OR || Left to right
Conditional ?: Right to left
Assignment = += -= *= /= %= >>= <<= &= ^= |= Right to left
Comma , Left to right
What is Next?
Next chapter would explain about loop control in Java programming. The chapter will describe various types of loops and how these loops can be used in Java program development and for what purposes they are being used
Java Modifier Types
Java Modifier Types
Modifiers are keywords that you add to those definitions to change their meanings. The Java language has a wide variety of modifiers, including the following:
Java Access Modifiers
Non Access Modifiers
To use a modifier, you include its keyword in the definition of a class, method, or variable. The modifier precedes the rest of the statement, as in the following examples (Italic ones):
public class className {
// ...
}
private boolean myFlag;
static final double weeks = 9.5;
protected static final int BOXWIDTH = 42;
public static void main(String[] arguments) {
// body of method
}
Access Control Modifiers:
Java provides a number of access modifiers to set access levels for classes, variables, methods and constructors. The four access levels are:
Visible to the package, the default. No modifiers are needed.
Visible to the class only (private).
Visible to the world (public).
Visible to the package and all subclasses (protected).
Non Access Modifiers:
Java provides a number of non-access modifiers to achieve many other functionality.
The static modifier for creating class methods and variables
The final modifier for finalizing the implementations of classes, methods, and variables.
The abstract modifier for creating abstract classes and methods.
The synchronized and volatile modifiers, which are used for threads.
What is Next?
In the next section, I will be discussing about Basic Operators used in the Java Language. The chapter will give you an overview of how these operators can be used during application development.
Modifiers are keywords that you add to those definitions to change their meanings. The Java language has a wide variety of modifiers, including the following:
Java Access Modifiers
Non Access Modifiers
To use a modifier, you include its keyword in the definition of a class, method, or variable. The modifier precedes the rest of the statement, as in the following examples (Italic ones):
public class className {
// ...
}
private boolean myFlag;
static final double weeks = 9.5;
protected static final int BOXWIDTH = 42;
public static void main(String[] arguments) {
// body of method
}
Access Control Modifiers:
Java provides a number of access modifiers to set access levels for classes, variables, methods and constructors. The four access levels are:
Visible to the package, the default. No modifiers are needed.
Visible to the class only (private).
Visible to the world (public).
Visible to the package and all subclasses (protected).
Non Access Modifiers:
Java provides a number of non-access modifiers to achieve many other functionality.
The static modifier for creating class methods and variables
The final modifier for finalizing the implementations of classes, methods, and variables.
The abstract modifier for creating abstract classes and methods.
The synchronized and volatile modifiers, which are used for threads.
What is Next?
In the next section, I will be discussing about Basic Operators used in the Java Language. The chapter will give you an overview of how these operators can be used during application development.
Java Variable Types
Java Variable Types
In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here:
type identifier [ = value][, identifier [= value] ...] ;
The type is one of Java's datatypes. The identifier is the name of the variable. To declare more than one variable of the specified type, use a comma-separated list.
Here are several examples of variable declarations of various types. Note that some include an initialization.
int a, b, c; // declares three ints, a, b, and c.
int d = 3, e, f = 5; // declares three more ints, initializing
// d and f.
byte z = 22; // initializes z.
double pi = 3.14159; // declares an approximation of pi.
char x = 'x'; // the variable x has the value 'x'.
This chapter will explain various variable types available in Java Language. There are three kinds of variables in Java:
Local variables
Instance variables
Class/static variables
Local variables:
Local variables are declared in methods, constructors, or blocks.
Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor or block.
Access modifiers cannot be used for local variables.
Local variables are visible only within the declared method, constructor or block.
Local variables are implemented at stack level internally.
There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.
Example:
Here, age is a local variable. This is defined inside pupAge() method and its scope is limited to this method only.
public class Test{
public void pupAge(){
int age = 0;
age = age + 7;
System.out.println("Puppy age is : " + age);
}
public static void main(String args[]){
Test test = new Test();
test.pupAge();
}
}
This would produce the following result:
Puppy age is: 7
Example:
Following example uses age without initializing it, so it would give an error at the time of compilation.
public class Test{
public void pupAge(){
int age;
age = age + 7;
System.out.println("Puppy age is : " + age);
}
public static void main(String args[]){
Test test = new Test();
test.pupAge();
}
}
This would produce the following error while compiling it:
Test.java:4:variable number might not have been initialized
age = age + 7;
^
1 error
Instance variables:
Instance variables are declared in a class, but outside a method, constructor or any block.
When a space is allocated for an object in the heap, a slot for each instance variable value is created.
Instance variables are created when an object is created with the use of the keyword 'new' and destroyed when the object is destroyed.
Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object's state that must be present throughout the class.
Instance variables can be declared in class level before or after use.
Access modifiers can be given for instance variables.
The instance variables are visible for all methods, constructors and block in the class. Normally, it is recommended to make these variables private (access level). However visibility for subclasses can be given for these variables with the use of access modifiers.
Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor.
Instance variables can be accessed directly by calling the variable name inside the class. However within static methods and different class ( when instance variables are given accessibility) should be called using the fully qualified name . ObjectReference.VariableName.
Example:
import java.io.*;
public class Employee{
// this instance variable is visible for any child class.
public String name;
// salary variable is visible in Employee class only.
private double salary;
// The name variable is assigned in the constructor.
public Employee (String empName){
name = empName;
}
// The salary variable is assigned a value.
public void setSalary(double empSal){
salary = empSal;
}
// This method prints the employee details.
public void printEmp(){
System.out.println("name : " + name );
System.out.println("salary :" + salary);
}
public static void main(String args[]){
Employee empOne = new Employee("Ransika");
empOne.setSalary(1000);
empOne.printEmp();
}
}
This would produce the following result:
name : Ransika
salary :1000.0
Class/static variables:
Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
There would only be one copy of each class variable per class, regardless of how many objects are created from it.
Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final and static. Constant variables never change from their initial value.
Static variables are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.
Static variables are created when the program starts and destroyed when the program stops.
Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
Default values are same as instance variables. For numbers, the default value is 0; for Booleans, it is false; and for object references, it is null. Values can be assigned during the declaration or within the constructor. Additionally values can be assigned in special static initializer blocks.
Static variables can be accessed by calling with the class name . ClassName.VariableName.
When declaring class variables as public static final, then variables names (constants) are all in upper case. If the static variables are not public and final the naming syntax is the same as instance and local variables.
Example:
import java.io.*;
public class Employee{
// salary variable is a private static variable
private static double salary;
// DEPARTMENT is a constant
public static final String DEPARTMENT = "Development ";
public static void main(String args[]){
salary = 1000;
System.out.println(DEPARTMENT+"average salary:"+salary);
}
}
This would produce the following result:
Development average salary:1000
Note: If the variables are access from an outside class the constant should be accessed as Employee.DEPARTMENT
What is Next?
You already have used access modifiers ( public & private ) in this chapter. The next chapter will explain you Access Modifiers and Non Access Modifiers in details
In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here:
type identifier [ = value][, identifier [= value] ...] ;
The type is one of Java's datatypes. The identifier is the name of the variable. To declare more than one variable of the specified type, use a comma-separated list.
Here are several examples of variable declarations of various types. Note that some include an initialization.
int a, b, c; // declares three ints, a, b, and c.
int d = 3, e, f = 5; // declares three more ints, initializing
// d and f.
byte z = 22; // initializes z.
double pi = 3.14159; // declares an approximation of pi.
char x = 'x'; // the variable x has the value 'x'.
This chapter will explain various variable types available in Java Language. There are three kinds of variables in Java:
Local variables
Instance variables
Class/static variables
Local variables:
Local variables are declared in methods, constructors, or blocks.
Local variables are created when the method, constructor or block is entered and the variable will be destroyed once it exits the method, constructor or block.
Access modifiers cannot be used for local variables.
Local variables are visible only within the declared method, constructor or block.
Local variables are implemented at stack level internally.
There is no default value for local variables so local variables should be declared and an initial value should be assigned before the first use.
Example:
Here, age is a local variable. This is defined inside pupAge() method and its scope is limited to this method only.
public class Test{
public void pupAge(){
int age = 0;
age = age + 7;
System.out.println("Puppy age is : " + age);
}
public static void main(String args[]){
Test test = new Test();
test.pupAge();
}
}
This would produce the following result:
Puppy age is: 7
Example:
Following example uses age without initializing it, so it would give an error at the time of compilation.
public class Test{
public void pupAge(){
int age;
age = age + 7;
System.out.println("Puppy age is : " + age);
}
public static void main(String args[]){
Test test = new Test();
test.pupAge();
}
}
This would produce the following error while compiling it:
Test.java:4:variable number might not have been initialized
age = age + 7;
^
1 error
Instance variables:
Instance variables are declared in a class, but outside a method, constructor or any block.
When a space is allocated for an object in the heap, a slot for each instance variable value is created.
Instance variables are created when an object is created with the use of the keyword 'new' and destroyed when the object is destroyed.
Instance variables hold values that must be referenced by more than one method, constructor or block, or essential parts of an object's state that must be present throughout the class.
Instance variables can be declared in class level before or after use.
Access modifiers can be given for instance variables.
The instance variables are visible for all methods, constructors and block in the class. Normally, it is recommended to make these variables private (access level). However visibility for subclasses can be given for these variables with the use of access modifiers.
Instance variables have default values. For numbers the default value is 0, for Booleans it is false and for object references it is null. Values can be assigned during the declaration or within the constructor.
Instance variables can be accessed directly by calling the variable name inside the class. However within static methods and different class ( when instance variables are given accessibility) should be called using the fully qualified name . ObjectReference.VariableName.
Example:
import java.io.*;
public class Employee{
// this instance variable is visible for any child class.
public String name;
// salary variable is visible in Employee class only.
private double salary;
// The name variable is assigned in the constructor.
public Employee (String empName){
name = empName;
}
// The salary variable is assigned a value.
public void setSalary(double empSal){
salary = empSal;
}
// This method prints the employee details.
public void printEmp(){
System.out.println("name : " + name );
System.out.println("salary :" + salary);
}
public static void main(String args[]){
Employee empOne = new Employee("Ransika");
empOne.setSalary(1000);
empOne.printEmp();
}
}
This would produce the following result:
name : Ransika
salary :1000.0
Class/static variables:
Class variables also known as static variables are declared with the static keyword in a class, but outside a method, constructor or a block.
There would only be one copy of each class variable per class, regardless of how many objects are created from it.
Static variables are rarely used other than being declared as constants. Constants are variables that are declared as public/private, final and static. Constant variables never change from their initial value.
Static variables are stored in static memory. It is rare to use static variables other than declared final and used as either public or private constants.
Static variables are created when the program starts and destroyed when the program stops.
Visibility is similar to instance variables. However, most static variables are declared public since they must be available for users of the class.
Default values are same as instance variables. For numbers, the default value is 0; for Booleans, it is false; and for object references, it is null. Values can be assigned during the declaration or within the constructor. Additionally values can be assigned in special static initializer blocks.
Static variables can be accessed by calling with the class name . ClassName.VariableName.
When declaring class variables as public static final, then variables names (constants) are all in upper case. If the static variables are not public and final the naming syntax is the same as instance and local variables.
Example:
import java.io.*;
public class Employee{
// salary variable is a private static variable
private static double salary;
// DEPARTMENT is a constant
public static final String DEPARTMENT = "Development ";
public static void main(String args[]){
salary = 1000;
System.out.println(DEPARTMENT+"average salary:"+salary);
}
}
This would produce the following result:
Development average salary:1000
Note: If the variables are access from an outside class the constant should be accessed as Employee.DEPARTMENT
What is Next?
You already have used access modifiers ( public & private ) in this chapter. The next chapter will explain you Access Modifiers and Non Access Modifiers in details
Java Basic data types
Java Basic Data Types
Previous Page Next Page
Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory.
Based on the data type of a variable, the operating system allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or characters in these variables.
There are two data types available in Java:
Primitive Data Types
Reference/Object Data Types
Primitive Data Types:
There are eight primitive data types supported by Java. Primitive data types are predefined by the language and named by a keyword. Let us now look into detail about the eight primitive data types.
byte:
Byte data type is an 8-bit signed two's complement integer.
Minimum value is -128 (-2^7)
Maximum value is 127 (inclusive)(2^7 -1)
Default value is 0
Byte data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.
Example: byte a = 100 , byte b = -50
short:
Short data type is a 16-bit signed two's complement integer.
Minimum value is -32,768 (-2^15)
Maximum value is 32,767 (inclusive) (2^15 -1)
Short data type can also be used to save memory as byte data type. A short is 2 times smaller than an int
Default value is 0.
Example: short s = 10000, short r = -20000
int:
Int data type is a 32-bit signed two's complement integer.
Minimum value is - 2,147,483,648.(-2^31)
Maximum value is 2,147,483,647(inclusive).(2^31 -1)
Int is generally used as the default data type for integral values unless there is a concern about memory.
The default value is 0.
Example: int a = 100000, int b = -200000
long:
Long data type is a 64-bit signed two's complement integer.
Minimum value is -9,223,372,036,854,775,808.(-2^63)
Maximum value is 9,223,372,036,854,775,807 (inclusive). (2^63 -1)
This type is used when a wider range than int is needed.
Default value is 0L.
Example: long a = 100000L, int b = -200000L
float:
Float data type is a single-precision 32-bit IEEE 754 floating point.
Float is mainly used to save memory in large arrays of floating point numbers.
Default value is 0.0f.
Float data type is never used for precise values such as currency.
Example: float f1 = 234.5f
double:
double data type is a double-precision 64-bit IEEE 754 floating point.
This data type is generally used as the default data type for decimal values, generally the default choice.
Double data type should never be used for precise values such as currency.
Default value is 0.0d.
Example: double d1 = 123.4
boolean:
boolean data type represents one bit of information.
There are only two possible values: true and false.
This data type is used for simple flags that track true/false conditions.
Default value is false.
Example: boolean one = true
char:
char data type is a single 16-bit Unicode character.
Minimum value is '\u0000' (or 0).
Maximum value is '\uffff' (or 65,535 inclusive).
Char data type is used to store any character.
Example: char letterA ='A'
Reference Data Types:
Reference variables are created using defined constructors of the classes. They are used to access objects. These variables are declared to be of a specific type that cannot be changed. For example, Employee, Puppy etc.
Class objects, and various type of array variables come under reference data type.
Default value of any reference variable is null.
A reference variable can be used to refer to any object of the declared type or any compatible type.
Example: Animal animal = new Animal("giraffe");
Java Literals:
A literal is a source code representation of a fixed value. They are represented directly in the code without any computation.
Literals can be assigned to any primitive type variable. For example:
byte a = 68;
char a = 'A'
byte, int, long, and short can be expressed in decimal(base 10), hexadecimal(base 16) or octal(base 8) number systems as well.
Prefix 0 is used to indicate octal and prefix 0x indicates hexadecimal adecimal(base 16) or octal(base 8) number systems as well.
Prefix 0 is used to indicate octal and prefix 0x indicates hexadecimal when using these number systems for literals. For example:
int decimal = 100;
int octal = 0144;
int hexa = 0x64;
String literals in Java are specified like they are in most other languages by enclosing a sequence of characters between a pair of double quotes. Examples of string literals are:
"Hello World"
"two\nlines"
"\"This is in quotes\""
String and char types of literals can contain any Unicode characters. For example:
char a = '\u0001';
String a = "\u0001";
Java language supports few special escape sequences for String and char literals as well. They are:
Notation Character represented
\n Newline (0x0a)
\r Carriage return (0x0d)
\f Formfeed (0x0c)
\b Backspace (0x08)
\s Space (0x20)
\t tab
\" Double quote
\' Single quote
\\ backslash
\ddd Octal character (ddd)
\uxxxx Hexadecimal UNICODE character (xxxx)
What is Next?
This chapter explained you various data types, next topic explains different variable types and their usage. This will give you a good understanding about how they can be used in the Java classes, interfaces, etc.
Previous Page Next Page
Variables are nothing but reserved memory locations to store values. This means that when you create a variable you reserve some space in memory.
Based on the data type of a variable, the operating system allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or characters in these variables.
There are two data types available in Java:
Primitive Data Types
Reference/Object Data Types
Primitive Data Types:
There are eight primitive data types supported by Java. Primitive data types are predefined by the language and named by a keyword. Let us now look into detail about the eight primitive data types.
byte:
Byte data type is an 8-bit signed two's complement integer.
Minimum value is -128 (-2^7)
Maximum value is 127 (inclusive)(2^7 -1)
Default value is 0
Byte data type is used to save space in large arrays, mainly in place of integers, since a byte is four times smaller than an int.
Example: byte a = 100 , byte b = -50
short:
Short data type is a 16-bit signed two's complement integer.
Minimum value is -32,768 (-2^15)
Maximum value is 32,767 (inclusive) (2^15 -1)
Short data type can also be used to save memory as byte data type. A short is 2 times smaller than an int
Default value is 0.
Example: short s = 10000, short r = -20000
int:
Int data type is a 32-bit signed two's complement integer.
Minimum value is - 2,147,483,648.(-2^31)
Maximum value is 2,147,483,647(inclusive).(2^31 -1)
Int is generally used as the default data type for integral values unless there is a concern about memory.
The default value is 0.
Example: int a = 100000, int b = -200000
long:
Long data type is a 64-bit signed two's complement integer.
Minimum value is -9,223,372,036,854,775,808.(-2^63)
Maximum value is 9,223,372,036,854,775,807 (inclusive). (2^63 -1)
This type is used when a wider range than int is needed.
Default value is 0L.
Example: long a = 100000L, int b = -200000L
float:
Float data type is a single-precision 32-bit IEEE 754 floating point.
Float is mainly used to save memory in large arrays of floating point numbers.
Default value is 0.0f.
Float data type is never used for precise values such as currency.
Example: float f1 = 234.5f
double:
double data type is a double-precision 64-bit IEEE 754 floating point.
This data type is generally used as the default data type for decimal values, generally the default choice.
Double data type should never be used for precise values such as currency.
Default value is 0.0d.
Example: double d1 = 123.4
boolean:
boolean data type represents one bit of information.
There are only two possible values: true and false.
This data type is used for simple flags that track true/false conditions.
Default value is false.
Example: boolean one = true
char:
char data type is a single 16-bit Unicode character.
Minimum value is '\u0000' (or 0).
Maximum value is '\uffff' (or 65,535 inclusive).
Char data type is used to store any character.
Example: char letterA ='A'
Reference Data Types:
Reference variables are created using defined constructors of the classes. They are used to access objects. These variables are declared to be of a specific type that cannot be changed. For example, Employee, Puppy etc.
Class objects, and various type of array variables come under reference data type.
Default value of any reference variable is null.
A reference variable can be used to refer to any object of the declared type or any compatible type.
Example: Animal animal = new Animal("giraffe");
Java Literals:
A literal is a source code representation of a fixed value. They are represented directly in the code without any computation.
Literals can be assigned to any primitive type variable. For example:
byte a = 68;
char a = 'A'
byte, int, long, and short can be expressed in decimal(base 10), hexadecimal(base 16) or octal(base 8) number systems as well.
Prefix 0 is used to indicate octal and prefix 0x indicates hexadecimal adecimal(base 16) or octal(base 8) number systems as well.
Prefix 0 is used to indicate octal and prefix 0x indicates hexadecimal when using these number systems for literals. For example:
int decimal = 100;
int octal = 0144;
int hexa = 0x64;
String literals in Java are specified like they are in most other languages by enclosing a sequence of characters between a pair of double quotes. Examples of string literals are:
"Hello World"
"two\nlines"
"\"This is in quotes\""
String and char types of literals can contain any Unicode characters. For example:
char a = '\u0001';
String a = "\u0001";
Java language supports few special escape sequences for String and char literals as well. They are:
Notation Character represented
\n Newline (0x0a)
\r Carriage return (0x0d)
\f Formfeed (0x0c)
\b Backspace (0x08)
\s Space (0x20)
\t tab
\" Double quote
\' Single quote
\\ backslash
\ddd Octal character (ddd)
\uxxxx Hexadecimal UNICODE character (xxxx)
What is Next?
This chapter explained you various data types, next topic explains different variable types and their usage. This will give you a good understanding about how they can be used in the Java classes, interfaces, etc.
Java - Objects and Classes
Java - Objects and Classes
Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java supports the following fundamental concepts:
Polymorphism
Inheritance
Encapsulation
Abstraction
Classes
Objects
Instance
Method
Message Parsing
In this chapter, we will look into the concepts Classes and Objects.
Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/blue print that describes the behaviors/states that object of its type support.
Objects in Java:
Let us now look deep into what are objects. If we consider the real-world we can find many objects around us, Cars, Dogs, Humans, etc. All these objects have a state and behavior.
If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging, running
If you compare the software object with a real world object, they have very similar characteristics.
Software objects also have a state and behavior. A software object's state is stored in fields and behavior is shown via methods.
So in software development, methods operate on the internal state of an object and the object-to-object communication is done via methods.
Classes in Java:
A class is a blue print from which individual objects are created.
A sample of a class is given below:
public class Dog{
String breed;
int age;
String color;
void barking(){
}
void hungry(){
}
void sleeping(){
}
}
A class can contain any of the following variable types.
Local variables: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.
Instance variables: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.
Class variables: Class variables are variables declared with in a class, outside any method, with the static keyword.
A class can have any number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.
Below mentioned are some of the important topics that need to be discussed when looking into classes of the Java Language.
Constructors:
When discussing about classes, one of the most important sub topic would be constructors. Every class has a constructor. If we do not explicitly write a constructor for a class the Java compiler builds a default constructor for that class.
Each time a new object is created, at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.
Example of a constructor is given below:
public class Puppy{
public puppy(){
}
public puppy(String name){
// This constructor has one parameter, name.
}
}
Java also supports Singleton Classes where you would be able to create only one instance of a class.
Creating an Object:
As mentioned previously, a class provides the blueprints for objects. So basically an object is created from a class. In Java, the new key word is used to create new objects.
There are three steps when creating an object from a class:
Declaration: A variable declaration with a variable name with an object type.
Instantiation: The 'new' key word is used to create the object.
Initialization: The 'new' keyword is followed by a call to a constructor. This call initializes the new object.
Example of creating an object is given below:
public class Puppy{
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public static void main(String []args){
// Following statement would create an object myPuppy
Puppy myPuppy = new Puppy( "tommy" );
}
}
If we compile and run the above program, then it would produce the following result:
Passed Name is :tommy
Accessing Instance Variables and Methods:
Instance variables and methods are accessed via created objects. To access an instance variable the fully qualified path should be as follows:
/* First create an object */
ObjectReference = new Constructor();
/* Now call a variable as follows */
ObjectReference.variableName;
/* Now you can call a class method as follows */
ObjectReference.MethodName();
Example:
This example explains how to access instance variables and methods of a class:
public class Puppy{
int puppyAge;
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public void setAge( int age ){
puppyAge = age;
}
public int getAge( ){
System.out.println("Puppy's age is :" + puppyAge );
return puppyAge;
}
public static void main(String []args){
/* Object creation */
Puppy myPuppy = new Puppy( "tommy" );
/* Call class method to set puppy's age */
myPuppy.setAge( 2 );
/* Call another class method to get puppy's age */
myPuppy.getAge( );
/* You can access instance variable as follows as well */
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
}
If we compile and run the above program, then it would produce the following result:
Passed Name is :tommy
Puppy's age is :2
Variable Value :2
Source file declaration rules:
As the last part of this section let's now look into the source file declaration rules. These rules are essential when declaring classes, import statements and package statements in a source file.
There can be only one public class per source file.
A source file can have multiple non public classes.
The public class name should be the name of the source file as well which should be appended by .java at the end. For example : The class name is . public class Employee{} Then the source file should be as Employee.java.
If the class is defined inside a package, then the package statement should be the first statement in the source file.
If import statements are present then they must be written between the package statement and the class declaration. If there are no package statements then the import statement should be the first line in the source file.
Import and package statements will imply to all the classes present in the source file. It is not possible to declare different import and/or package statements to different classes in the source file.
Classes have several access levels and there are different types of classes; abstract classes, final classes, etc. I will be explaining about all these in the access modifiers chapter.
Apart from the above mentioned types of classes, Java also has some special classes called Inner classes and Anonymous classes.
Java Package:
In simple, it is a way of categorizing the classes and interfaces. When developing applications in Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a must as well as makes life much easier.
Import statements:
In Java if a fully qualified name, which includes the package and the class name, is given then the compiler can easily locate the source code or classes. Import statement is a way of giving the proper location for the compiler to find that particular class.
For example, the following line would ask compiler to load all the classes available in directory java_installation/java/io :
import java.io.*;
A Simple Case Study:
For our case study, we will be creating two classes. They are Employee and EmployeeTest.
First open notepad and add the following code. Remember this is the Employee class and the class is a public class. Now, save this source file with the name Employee.java.
The Employee class has four instance variables name, age, designation and salary. The class has one explicitly defined constructor, which takes a parameter.
import java.io.*;
public class Employee{
String name;
int age;
String designation;
double salary;
// This is the constructor of the class Employee
public Employee(String name){
this.name = name;
}
// Assign the age of the Employee to the variable age.
public void empAge(int empAge){
age = empAge;
}
/* Assign the designation to the variable designation.*/
public void empDesignation(String empDesig){
designation = empDesig;
}
/* Assign the salary to the variable salary.*/
public void empSalary(double empSalary){
salary = empSalary;
}
/* Print the Employee details */
public void printEmployee(){
System.out.println("Name:"+ name );
System.out.println("Age:" + age );
System.out.println("Designation:" + designation );
System.out.println("Salary:" + salary);
}
}
As mentioned previously in this tutorial, processing starts from the main method. Therefore in-order for us to run this Employee class there should be main method and objects should be created. We will be creating a separate class for these tasks.
Given below is the EmployeeTest class, which creates two instances of the class Employee and invokes the methods for each object to assign values for each variable.
Save the following code in EmployeeTest.java file
import java.io.*;public class EmployeeTest{
public static void main(String args[]){
/* Create two objects using constructor */
Employee empOne = new Employee("James Smith");
Employee empTwo = new Employee("Mary Anne");
// Invoking methods for each object created
empOne.empAge(26);
empOne.empDesignation("Senior Software Engineer");
empOne.empSalary(1000);
empOne.printEmployee();
empTwo.empAge(21);
empTwo.empDesignation("Software Engineer");
empTwo.empSalary(500);
empTwo.printEmployee();
}
}
Now, compile both the classes and then run EmployeeTest to see the result as follows:
C :> javac Employee.java
C :> vi EmployeeTest.java
C :> javac EmployeeTest.java
C :> java EmployeeTest
Name:James Smith
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Mary Anne
Age:21
Designation:Software Engineer
Salary:500.0
What is Next?
Next session will discuss basic data types in Java and how they can be used when developing Java applications.
Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java supports the following fundamental concepts:
Polymorphism
Inheritance
Encapsulation
Abstraction
Classes
Objects
Instance
Method
Message Parsing
In this chapter, we will look into the concepts Classes and Objects.
Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/blue print that describes the behaviors/states that object of its type support.
Objects in Java:
Let us now look deep into what are objects. If we consider the real-world we can find many objects around us, Cars, Dogs, Humans, etc. All these objects have a state and behavior.
If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging, running
If you compare the software object with a real world object, they have very similar characteristics.
Software objects also have a state and behavior. A software object's state is stored in fields and behavior is shown via methods.
So in software development, methods operate on the internal state of an object and the object-to-object communication is done via methods.
Classes in Java:
A class is a blue print from which individual objects are created.
A sample of a class is given below:
public class Dog{
String breed;
int age;
String color;
void barking(){
}
void hungry(){
}
void sleeping(){
}
}
A class can contain any of the following variable types.
Local variables: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.
Instance variables: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.
Class variables: Class variables are variables declared with in a class, outside any method, with the static keyword.
A class can have any number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.
Below mentioned are some of the important topics that need to be discussed when looking into classes of the Java Language.
Constructors:
When discussing about classes, one of the most important sub topic would be constructors. Every class has a constructor. If we do not explicitly write a constructor for a class the Java compiler builds a default constructor for that class.
Each time a new object is created, at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.
Example of a constructor is given below:
public class Puppy{
public puppy(){
}
public puppy(String name){
// This constructor has one parameter, name.
}
}
Java also supports Singleton Classes where you would be able to create only one instance of a class.
Creating an Object:
As mentioned previously, a class provides the blueprints for objects. So basically an object is created from a class. In Java, the new key word is used to create new objects.
There are three steps when creating an object from a class:
Declaration: A variable declaration with a variable name with an object type.
Instantiation: The 'new' key word is used to create the object.
Initialization: The 'new' keyword is followed by a call to a constructor. This call initializes the new object.
Example of creating an object is given below:
public class Puppy{
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public static void main(String []args){
// Following statement would create an object myPuppy
Puppy myPuppy = new Puppy( "tommy" );
}
}
If we compile and run the above program, then it would produce the following result:
Passed Name is :tommy
Accessing Instance Variables and Methods:
Instance variables and methods are accessed via created objects. To access an instance variable the fully qualified path should be as follows:
/* First create an object */
ObjectReference = new Constructor();
/* Now call a variable as follows */
ObjectReference.variableName;
/* Now you can call a class method as follows */
ObjectReference.MethodName();
Example:
This example explains how to access instance variables and methods of a class:
public class Puppy{
int puppyAge;
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public void setAge( int age ){
puppyAge = age;
}
public int getAge( ){
System.out.println("Puppy's age is :" + puppyAge );
return puppyAge;
}
public static void main(String []args){
/* Object creation */
Puppy myPuppy = new Puppy( "tommy" );
/* Call class method to set puppy's age */
myPuppy.setAge( 2 );
/* Call another class method to get puppy's age */
myPuppy.getAge( );
/* You can access instance variable as follows as well */
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
}
If we compile and run the above program, then it would produce the following result:
Passed Name is :tommy
Puppy's age is :2
Variable Value :2
Source file declaration rules:
As the last part of this section let's now look into the source file declaration rules. These rules are essential when declaring classes, import statements and package statements in a source file.
There can be only one public class per source file.
A source file can have multiple non public classes.
The public class name should be the name of the source file as well which should be appended by .java at the end. For example : The class name is . public class Employee{} Then the source file should be as Employee.java.
If the class is defined inside a package, then the package statement should be the first statement in the source file.
If import statements are present then they must be written between the package statement and the class declaration. If there are no package statements then the import statement should be the first line in the source file.
Import and package statements will imply to all the classes present in the source file. It is not possible to declare different import and/or package statements to different classes in the source file.
Classes have several access levels and there are different types of classes; abstract classes, final classes, etc. I will be explaining about all these in the access modifiers chapter.
Apart from the above mentioned types of classes, Java also has some special classes called Inner classes and Anonymous classes.
Java Package:
In simple, it is a way of categorizing the classes and interfaces. When developing applications in Java, hundreds of classes and interfaces will be written, therefore categorizing these classes is a must as well as makes life much easier.
Import statements:
In Java if a fully qualified name, which includes the package and the class name, is given then the compiler can easily locate the source code or classes. Import statement is a way of giving the proper location for the compiler to find that particular class.
For example, the following line would ask compiler to load all the classes available in directory java_installation/java/io :
import java.io.*;
A Simple Case Study:
For our case study, we will be creating two classes. They are Employee and EmployeeTest.
First open notepad and add the following code. Remember this is the Employee class and the class is a public class. Now, save this source file with the name Employee.java.
The Employee class has four instance variables name, age, designation and salary. The class has one explicitly defined constructor, which takes a parameter.
import java.io.*;
public class Employee{
String name;
int age;
String designation;
double salary;
// This is the constructor of the class Employee
public Employee(String name){
this.name = name;
}
// Assign the age of the Employee to the variable age.
public void empAge(int empAge){
age = empAge;
}
/* Assign the designation to the variable designation.*/
public void empDesignation(String empDesig){
designation = empDesig;
}
/* Assign the salary to the variable salary.*/
public void empSalary(double empSalary){
salary = empSalary;
}
/* Print the Employee details */
public void printEmployee(){
System.out.println("Name:"+ name );
System.out.println("Age:" + age );
System.out.println("Designation:" + designation );
System.out.println("Salary:" + salary);
}
}
As mentioned previously in this tutorial, processing starts from the main method. Therefore in-order for us to run this Employee class there should be main method and objects should be created. We will be creating a separate class for these tasks.
Given below is the EmployeeTest class, which creates two instances of the class Employee and invokes the methods for each object to assign values for each variable.
Save the following code in EmployeeTest.java file
import java.io.*;public class EmployeeTest{
public static void main(String args[]){
/* Create two objects using constructor */
Employee empOne = new Employee("James Smith");
Employee empTwo = new Employee("Mary Anne");
// Invoking methods for each object created
empOne.empAge(26);
empOne.empDesignation("Senior Software Engineer");
empOne.empSalary(1000);
empOne.printEmployee();
empTwo.empAge(21);
empTwo.empDesignation("Software Engineer");
empTwo.empSalary(500);
empTwo.printEmployee();
}
}
Now, compile both the classes and then run EmployeeTest to see the result as follows:
C :> javac Employee.java
C :> vi EmployeeTest.java
C :> javac EmployeeTest.java
C :> java EmployeeTest
Name:James Smith
Age:26
Designation:Senior Software Engineer
Salary:1000.0
Name:Mary Anne
Age:21
Designation:Software Engineer
Salary:500.0
What is Next?
Next session will discuss basic data types in Java and how they can be used when developing Java applications.
Java Basic Syntax
Java Basic Syntax
When we consider a Java program it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean.
Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/ blue print that describes the behaviors/states that object of its type support.
Methods - A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.
First Java Program:
Let us look at a simple code that would print the words Hello World.
public class MyFirstJavaProgram {
/* This is my first java program.
* This will print 'Hello World' as the output
*/
public static void main(String []args) {
System.out.println("Hello World"); // prints Hello World
}
}
Let's look at how to save the file, compile and run the program. Please follow the steps given below:
Open notepad and add the code as above.
Save the file as: MyFirstJavaProgram.java.
Open a command prompt window and go o the directory where you saved the class. Assume it's C:\.
Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).
Now, type ' java MyFirstJavaProgram ' to run your program.
You will be able to see ' Hello World ' printed on the window.
C : > javac MyFirstJavaProgram.java
C : > java MyFirstJavaProgram
Hello World
Basic Syntax:
About Java programs, it is very important to keep in mind the following points.
Case Sensitivity - Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
Class Names - For all class names the first letter should be in Upper Case.
If several words are used to form a name of the class, each inner word's first letter should be in Upper Case.
Example class MyFirstJavaClass
Method Names - All method names should start with a Lower Case letter.
If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case.
Example public void myMethodName()
Program File Name - Name of the program file should exactly match the class name.
When saving the file, you should save it using the class name (Remember Java is case sensitive) and append '.java' to the end of the name (if the file name and the class name do not match your program will not compile).
Example : Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as 'MyFirstJavaProgram.java'
public static void main(String args[]) - Java program processing starts from the main() method which is a mandatory part of every Java program..
Java Identifiers:
All Java components require names. Names used for classes, variables and methods are called identifiers.
In Java, there are several points to remember about identifiers. They are as follows:
All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
After the first character identifiers can have any combination of characters.
A key word cannot be used as an identifier.
Most importantly identifiers are case sensitive.
Examples of legal identifiers: age, $salary, _value, __1_value
Examples of illegal identifiers: 123abc, -salary
Java Modifiers:
Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers:
Access Modifiers: default, public , protected, private
Non-access Modifiers: final, abstract, strictfp
We will be looking into more details about modifiers in the next section.
Java Variables:
We would see folWe would see following type of variables in Java:
Local Variables
Class Variables (Static Variables)
Instance Variables (Non-static variables)
Java Arrays:
Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct and initialize in the upcoming chapters.
Java Enums:
Enums were introduced in java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.
With the use of enums it is possible to reduce the number of bugs in your code.
For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium and large. This would make sure that it would not allow anyone to order any size other than the small, medium or large.
Example:
class FreshJuice {
enum FreshJuiceSize{ SMALL, MEDIUM, LARGE }
FreshJuiceSize size;
}
public class FreshJuiceTest {
public static void main(String args[]){
FreshJuice juice = new FreshJuice();
juice.size = FreshJuice. FreshJuiceSize.MEDIUM ;
System.out.println("Size: " + juice.size);
}
}
Above example will produce the following result:
Size: MEDIUM
Note: enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.
Java Keywords:
The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.
abstract assert boolean break
byte case catch char
class const continue default
do double else enum
extends final finally float
for goto if implements
import instanceof int interface
long native new package
private protected public return
short static strictfp super
switch synchronized this throw
throws transient try void
volatile while
Comments in Java
Java supports single-line and multi-line comments very similar to c and c++. All characters available inside any comment are ignored by Java compiler.
public class MyFirstJavaProgram{
/* This is my first java program.
* This will print 'Hello World' as the output
* This is an example of multi-line comments.
*/
public static void main(String []args){
// This is an example of single line comment
/* This is also an example of single line comment. */
System.out.println("Hello World");
}
}
Using Blank Lines:
A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.
Inheritance:
In Java, classes can be derived from classes. Basically if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.
This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario the existing class is called the superclass and the derived class is called the subclass.
Interfaces:
In Java language, an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.
An interface defines the methods, a deriving class(subclass) should use. But the implementation of the methods is totally up to the subclass.
What is Next?
The next section explains about Objects and classes in Java programming. At the end of the session you will be able to get a clear picture as to what are objects and what are classes in Java.
When we consider a Java program it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean.
Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/ blue print that describes the behaviors/states that object of its type support.
Methods - A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.
First Java Program:
Let us look at a simple code that would print the words Hello World.
public class MyFirstJavaProgram {
/* This is my first java program.
* This will print 'Hello World' as the output
*/
public static void main(String []args) {
System.out.println("Hello World"); // prints Hello World
}
}
Let's look at how to save the file, compile and run the program. Please follow the steps given below:
Open notepad and add the code as above.
Save the file as: MyFirstJavaProgram.java.
Open a command prompt window and go o the directory where you saved the class. Assume it's C:\.
Type ' javac MyFirstJavaProgram.java ' and press enter to compile your code. If there are no errors in your code, the command prompt will take you to the next line (Assumption : The path variable is set).
Now, type ' java MyFirstJavaProgram ' to run your program.
You will be able to see ' Hello World ' printed on the window.
C : > javac MyFirstJavaProgram.java
C : > java MyFirstJavaProgram
Hello World
Basic Syntax:
About Java programs, it is very important to keep in mind the following points.
Case Sensitivity - Java is case sensitive, which means identifier Hello and hello would have different meaning in Java.
Class Names - For all class names the first letter should be in Upper Case.
If several words are used to form a name of the class, each inner word's first letter should be in Upper Case.
Example class MyFirstJavaClass
Method Names - All method names should start with a Lower Case letter.
If several words are used to form the name of the method, then each inner word's first letter should be in Upper Case.
Example public void myMethodName()
Program File Name - Name of the program file should exactly match the class name.
When saving the file, you should save it using the class name (Remember Java is case sensitive) and append '.java' to the end of the name (if the file name and the class name do not match your program will not compile).
Example : Assume 'MyFirstJavaProgram' is the class name. Then the file should be saved as 'MyFirstJavaProgram.java'
public static void main(String args[]) - Java program processing starts from the main() method which is a mandatory part of every Java program..
Java Identifiers:
All Java components require names. Names used for classes, variables and methods are called identifiers.
In Java, there are several points to remember about identifiers. They are as follows:
All identifiers should begin with a letter (A to Z or a to z), currency character ($) or an underscore (_).
After the first character identifiers can have any combination of characters.
A key word cannot be used as an identifier.
Most importantly identifiers are case sensitive.
Examples of legal identifiers: age, $salary, _value, __1_value
Examples of illegal identifiers: 123abc, -salary
Java Modifiers:
Like other languages, it is possible to modify classes, methods, etc., by using modifiers. There are two categories of modifiers:
Access Modifiers: default, public , protected, private
Non-access Modifiers: final, abstract, strictfp
We will be looking into more details about modifiers in the next section.
Java Variables:
We would see folWe would see following type of variables in Java:
Local Variables
Class Variables (Static Variables)
Instance Variables (Non-static variables)
Java Arrays:
Arrays are objects that store multiple variables of the same type. However, an array itself is an object on the heap. We will look into how to declare, construct and initialize in the upcoming chapters.
Java Enums:
Enums were introduced in java 5.0. Enums restrict a variable to have one of only a few predefined values. The values in this enumerated list are called enums.
With the use of enums it is possible to reduce the number of bugs in your code.
For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium and large. This would make sure that it would not allow anyone to order any size other than the small, medium or large.
Example:
class FreshJuice {
enum FreshJuiceSize{ SMALL, MEDIUM, LARGE }
FreshJuiceSize size;
}
public class FreshJuiceTest {
public static void main(String args[]){
FreshJuice juice = new FreshJuice();
juice.size = FreshJuice. FreshJuiceSize.MEDIUM ;
System.out.println("Size: " + juice.size);
}
}
Above example will produce the following result:
Size: MEDIUM
Note: enums can be declared as their own or inside a class. Methods, variables, constructors can be defined inside enums as well.
Java Keywords:
The following list shows the reserved words in Java. These reserved words may not be used as constant or variable or any other identifier names.
abstract assert boolean break
byte case catch char
class const continue default
do double else enum
extends final finally float
for goto if implements
import instanceof int interface
long native new package
private protected public return
short static strictfp super
switch synchronized this throw
throws transient try void
volatile while
Comments in Java
Java supports single-line and multi-line comments very similar to c and c++. All characters available inside any comment are ignored by Java compiler.
public class MyFirstJavaProgram{
/* This is my first java program.
* This will print 'Hello World' as the output
* This is an example of multi-line comments.
*/
public static void main(String []args){
// This is an example of single line comment
/* This is also an example of single line comment. */
System.out.println("Hello World");
}
}
Using Blank Lines:
A line containing only whitespace, possibly with a comment, is known as a blank line, and Java totally ignores it.
Inheritance:
In Java, classes can be derived from classes. Basically if you need to create a new class and here is already a class that has some of the code you require, then it is possible to derive your new class from the already existing code.
This concept allows you to reuse the fields and methods of the existing class without having to rewrite the code in a new class. In this scenario the existing class is called the superclass and the derived class is called the subclass.
Interfaces:
In Java language, an interface can be defined as a contract between objects on how to communicate with each other. Interfaces play a vital role when it comes to the concept of inheritance.
An interface defines the methods, a deriving class(subclass) should use. But the implementation of the methods is totally up to the subclass.
What is Next?
The next section explains about Objects and classes in Java programming. At the end of the session you will be able to get a clear picture as to what are objects and what are classes in Java.
Java Environment Setup
Java Environment Setup
You really do not need to set up your own environment to start learning Java programming language. Reason is very simple, we already have setup Java Programming environment online, so that you can compile and execute all the available examples online at the same time when you are doing your theory work. This gives you confidence in what you are reading and to check the result with different options. Feel free to modify any example and execute it online.
Try following example using Try it option available at the top right corner of the below sample code box:
public class MyFirstJavaProgram {
public static void main(String []args) {
System.out.println("Hello World");
}
}
For most of the examples given in this tutorial, you will find Try it option, so just make use of it and enjoy your learning.
Local Environment Setup
If you are still willing to set up your environment for Java programming language, then this section guides you on how to download and set up Java on your machine. Please follow the following steps to set up the environment.
Java SE is freely available from the link Download Java. So you download a version based on your operating system.
Follow the instructions to download java and run the .exe to install Java on your machine. Once you installed Java on your machine, you would need to set environment variables to point to correct installation directories:
Setting up the path for windows 2000/XP:
Assuming you have installed Java in c:\Program Files\java\jdk directory:
Right-click on 'My Computer' and select 'Properties'.
Click on the 'Environment variables' button under the 'Advanced' tab.
Now, alter the 'Path' variable so that it also contains the path to the Java executable. Example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change your path to read 'C:\WINDOWS\SYSTEM32;c:\Program Files\java\jdk\bin'.
Setting up the path for windows 95/98/ME:
Assuming you have installed Java in c:\Program Files\java\jdk directory:
Edit the 'C:\autoexec.bat' file and add the following line at the end:
'SET PATH=%PATH%;C:\Program Files\java\jdk\bin'
Setting up the path for Linux, UNIX, Solaris, FreeBSD:
Environment variable PATH should be set to point to where the Java binaries have been installed. Refer to your shell documentation if you have trouble doing this.
Example, if you use bash as your shell, then you would add the following line to the end of your '.bashrc: export PATH=/path/to/java:$PATH'
Popular Java Editors:
To write your Java programs, you will need a text editor. There are even more sophisticated IDEs available in the market. But for now, you can consider one of the following:
Notepad: On Windows machine you can use any simple text editor like Notepad (Recommended for this tutorial), TextPad.
Netbeans:is a Java IDE that is open-source and free which can be downloaded from http://www.netbeans.org/index.html.
Eclipse: is also a Java IDE developed by the eclipse open-source community and can be downloaded from http://www.eclipse.org/.
What is Next?
Next chapter will teach you how to write and run your first Java program and some of the important basic syntaxes in Java needed for developing applications.
You really do not need to set up your own environment to start learning Java programming language. Reason is very simple, we already have setup Java Programming environment online, so that you can compile and execute all the available examples online at the same time when you are doing your theory work. This gives you confidence in what you are reading and to check the result with different options. Feel free to modify any example and execute it online.
Try following example using Try it option available at the top right corner of the below sample code box:
public class MyFirstJavaProgram {
public static void main(String []args) {
System.out.println("Hello World");
}
}
For most of the examples given in this tutorial, you will find Try it option, so just make use of it and enjoy your learning.
Local Environment Setup
If you are still willing to set up your environment for Java programming language, then this section guides you on how to download and set up Java on your machine. Please follow the following steps to set up the environment.
Java SE is freely available from the link Download Java. So you download a version based on your operating system.
Follow the instructions to download java and run the .exe to install Java on your machine. Once you installed Java on your machine, you would need to set environment variables to point to correct installation directories:
Setting up the path for windows 2000/XP:
Assuming you have installed Java in c:\Program Files\java\jdk directory:
Right-click on 'My Computer' and select 'Properties'.
Click on the 'Environment variables' button under the 'Advanced' tab.
Now, alter the 'Path' variable so that it also contains the path to the Java executable. Example, if the path is currently set to 'C:\WINDOWS\SYSTEM32', then change your path to read 'C:\WINDOWS\SYSTEM32;c:\Program Files\java\jdk\bin'.
Setting up the path for windows 95/98/ME:
Assuming you have installed Java in c:\Program Files\java\jdk directory:
Edit the 'C:\autoexec.bat' file and add the following line at the end:
'SET PATH=%PATH%;C:\Program Files\java\jdk\bin'
Setting up the path for Linux, UNIX, Solaris, FreeBSD:
Environment variable PATH should be set to point to where the Java binaries have been installed. Refer to your shell documentation if you have trouble doing this.
Example, if you use bash as your shell, then you would add the following line to the end of your '.bashrc: export PATH=/path/to/java:$PATH'
Popular Java Editors:
To write your Java programs, you will need a text editor. There are even more sophisticated IDEs available in the market. But for now, you can consider one of the following:
Notepad: On Windows machine you can use any simple text editor like Notepad (Recommended for this tutorial), TextPad.
Netbeans:is a Java IDE that is open-source and free which can be downloaded from http://www.netbeans.org/index.html.
Eclipse: is also a Java IDE developed by the eclipse open-source community and can be downloaded from http://www.eclipse.org/.
What is Next?
Next chapter will teach you how to write and run your first Java program and some of the important basic syntaxes in Java needed for developing applications.
Java - Overview
Java - Overview
Java programming language was originally developed by Sun Microsystems which was initiated by James Gosling and released in 1995 as core component of Sun Microsystems' Java platform (Java 1.0 [J2SE]).
As of December 2008, the latest release of the Java Standard Edition is 6 (J2SE). With the advancement of Java and its widespread popularity, multiple configurations were built to suite various types of platforms. Ex: J2EE for Enterprise Applications, J2ME for Mobile Applications.
Sun Microsystems has renamed the new J2 versions as Java SE, Java EE and Java ME respectively. Java is guaranteed to be Write Once, Run Anywhere.
Java is:
Object Oriented: In Java, everything is an Object. Java can be easily extended since it is based on the Object model.
Platform independent: Unlike many other programming languages including C and C++, when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.
Simple:Java is designed to be easy to learn. If you understand the basic concept of OOP Java would be easy to master.
Secure: With Java's secure feature it enables to develop virus-free, tamper-free systems. Authentication techniques are based on public-key encryption.
Architectural-neutral :Java compiler generates an architecture-neutral object file format which makes the compiled code to be executable on many processors, with the presence of Java runtime system.
Portable:Being architectural-neutral and having no implementation dependent aspects of the specification makes Java portable. Compiler in Java is written in ANSI C with a clean portability boundary which is a POSIX subset.
Robust:Java makes an effort to eliminate error prone situations by emphasizing mainly on compile time error checking and runtime checking.
Multithreaded: With Java's multithreaded feature it is possible to write programs that can do many tasks simultaneously. This design feature allows developers to construct smoothly running interactive applications.
Interpreted:Java byte code is translated on the fly to native machine instructions and is not stored anywhere. The development process is more rapid and analytical since the linking is an incremental and light weight process.
High Performance: With the use of Just-In-Time compilers, Java enables high performance.
Distributed:Java is designed for the distributed environment of the internet.
Dynamic: Java is considered to be more dynamic than C or C++ since it is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.
History of Java:
James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects. The language, initially called Oak after an oak tree that stood outside Gosling's office, also went by the name Green and ended up later being renamed as Java, from a list of random words.
Sun released the first public implementation as Java 1.0 in 1995. It promised Write Once, Run Anywhere (WORA), providing no-cost run-times on popular platforms.
On 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL).
On 8 May 2007, Sun finished the process, making all of Java's core code free and open-source, aside from a small portion of code to which Sun did not hold the copyright.
Tools you will need:
For performing the examples discussed in this tutorial, you will need a Pentium 200-MHz computer with a minimum of 64 MB of RAM (128 MB of RAM recommended).
You also will need the following softwares:
Linux 7.1 or Windows 95/98/2000/XP operating system.
Java JDK 5
Java JDK 5
Microsoft Notepad or any other text editor
This tutorial will provide the necessary skills to create GUI, networking, and Web applications using Java.
Try It Option:
We have provided you an option to compile and execute available code online. Just click on Try it button avaiable at top-right corner of the code window to compile and execute available code. There are certain examples which can not be executed online, so we have skipped those examples.
public class MyFirstJavaProgram {
public static void main(String []args) {
System.out.println("Hello World");
}
}
There may be a case that you do not see the result of the compiled/executed code, in such case you can re-try to compile and execute the code using execute button available in compliation pop-up window.
What is Next?
Next chapter will guide you to where you can obtain Java and its documentation. Finally, it instructs you on how to install Java and prepare an environment to develop Java applications.
Java programming language was originally developed by Sun Microsystems which was initiated by James Gosling and released in 1995 as core component of Sun Microsystems' Java platform (Java 1.0 [J2SE]).
As of December 2008, the latest release of the Java Standard Edition is 6 (J2SE). With the advancement of Java and its widespread popularity, multiple configurations were built to suite various types of platforms. Ex: J2EE for Enterprise Applications, J2ME for Mobile Applications.
Sun Microsystems has renamed the new J2 versions as Java SE, Java EE and Java ME respectively. Java is guaranteed to be Write Once, Run Anywhere.
Java is:
Object Oriented: In Java, everything is an Object. Java can be easily extended since it is based on the Object model.
Platform independent: Unlike many other programming languages including C and C++, when Java is compiled, it is not compiled into platform specific machine, rather into platform independent byte code. This byte code is distributed over the web and interpreted by virtual Machine (JVM) on whichever platform it is being run.
Simple:Java is designed to be easy to learn. If you understand the basic concept of OOP Java would be easy to master.
Secure: With Java's secure feature it enables to develop virus-free, tamper-free systems. Authentication techniques are based on public-key encryption.
Architectural-neutral :Java compiler generates an architecture-neutral object file format which makes the compiled code to be executable on many processors, with the presence of Java runtime system.
Portable:Being architectural-neutral and having no implementation dependent aspects of the specification makes Java portable. Compiler in Java is written in ANSI C with a clean portability boundary which is a POSIX subset.
Robust:Java makes an effort to eliminate error prone situations by emphasizing mainly on compile time error checking and runtime checking.
Multithreaded: With Java's multithreaded feature it is possible to write programs that can do many tasks simultaneously. This design feature allows developers to construct smoothly running interactive applications.
Interpreted:Java byte code is translated on the fly to native machine instructions and is not stored anywhere. The development process is more rapid and analytical since the linking is an incremental and light weight process.
High Performance: With the use of Just-In-Time compilers, Java enables high performance.
Distributed:Java is designed for the distributed environment of the internet.
Dynamic: Java is considered to be more dynamic than C or C++ since it is designed to adapt to an evolving environment. Java programs can carry extensive amount of run-time information that can be used to verify and resolve accesses to objects on run-time.
History of Java:
James Gosling initiated the Java language project in June 1991 for use in one of his many set-top box projects. The language, initially called Oak after an oak tree that stood outside Gosling's office, also went by the name Green and ended up later being renamed as Java, from a list of random words.
Sun released the first public implementation as Java 1.0 in 1995. It promised Write Once, Run Anywhere (WORA), providing no-cost run-times on popular platforms.
On 13 November 2006, Sun released much of Java as free and open source software under the terms of the GNU General Public License (GPL).
On 8 May 2007, Sun finished the process, making all of Java's core code free and open-source, aside from a small portion of code to which Sun did not hold the copyright.
Tools you will need:
For performing the examples discussed in this tutorial, you will need a Pentium 200-MHz computer with a minimum of 64 MB of RAM (128 MB of RAM recommended).
You also will need the following softwares:
Linux 7.1 or Windows 95/98/2000/XP operating system.
Java JDK 5
Java JDK 5
Microsoft Notepad or any other text editor
This tutorial will provide the necessary skills to create GUI, networking, and Web applications using Java.
Try It Option:
We have provided you an option to compile and execute available code online. Just click on Try it button avaiable at top-right corner of the code window to compile and execute available code. There are certain examples which can not be executed online, so we have skipped those examples.
public class MyFirstJavaProgram {
public static void main(String []args) {
System.out.println("Hello World");
}
}
There may be a case that you do not see the result of the compiled/executed code, in such case you can re-try to compile and execute the code using execute button available in compliation pop-up window.
What is Next?
Next chapter will guide you to where you can obtain Java and its documentation. Finally, it instructs you on how to install Java and prepare an environment to develop Java applications.
Monday, 30 December 2013
lessons 7.3Allowing Other Apps to Start Your Activity
Allowing Other Apps to Start Your Activity
The previous two lessons focused on one side of the story: starting another app's activity from your app. But if your app can perform an action that might be useful to another app, your app should be prepared to respond to action requests from other apps. For instance, if you build a social app that can share messages or photos with the user's friends, it's in your best interest to support the ACTION_SEND intent so users can initiate a "share" action from another app and launch your app to perform the action.
To allow other apps to start your activity, you need to add an element in your manifest file for the corresponding element.
When your app is installed on a device, the system identifies your intent filters and adds the information to an internal catalog of intents supported by all installed apps. When an app calls startActivity() or startActivityForResult(), with an implicit intent, the system finds which activity (or activities) can respond to the intent.
Add an Intent Filter
In order to properly define which intents your activity can handle, each intent filter you add should be as specific as possible in terms of the type of action and data the activity accepts.
The system may send a given Intent to an activity if that activity has an intent filter fulfills the following criteria of the Intent object:
Action
A string naming the action to perform. Usually one of the platform-defined values such as ACTION_SEND or ACTION_VIEW.
Specify this in your intent filter with the element. The value you specify in this element must be the full string name for the action, instead of the API constant (see the examples below).
Data
A description of the data associated with the intent.
Specify this in your intent filter with the element. Using one or more attributes in this element, you can specify just the MIME type, just a URI prefix, just a URI scheme, or a combination of these and others that indicate the data type accepted.
Note: If you don't need to declare specifics about the data Uri (such as when your activity handles to other kind of "extra" data, instead of a URI), you should specify only the android:mimeType attribute to declare the type of data your activity handles, such as text/plain or image/jpeg.
Category
Provides an additional way to characterize the activity handling the intent, usually related to the user gesture or location from which it's started. There are several different categories supported by the system, but most are rarely used. However, all implicit intents are defined with CATEGORY_DEFAULT by default.
Specify this in your intent filter with the element.
In your intent filter, you can declare which criteria your activity accepts by declaring each of them with corresponding XML elements nested in the element.
For example, here's an activity with an intent filter that handles the ACTION_SEND intent when the data type is either text or an image:
Each incoming intent specifies only one action and one data type, but it's OK to declare multiple instances of the, , and elements in each .
Each incoming intent specifies only one action and one data type, but it's OK to declare multiple instances of the, , and elements in each .
If any two pairs of action and data are mutually exclusive in their behaviors, you should create separate intent filters to specify which actions are acceptable when paired with which data types.
For example, suppose your activity handles both text and images for both the ACTION_SEND and ACTION_SENDTO intents. In this case, you must define two separate intent filters for the two actions because a ACTION_SENDTO intent must use the data Uri to specify the recipient's address using the send or sendto URI scheme. For example:
Note: In order to receive implicit intents, you must include the CATEGORY_DEFAULT category in the intent filter. The methods startActivity() and startActivityForResult() treat all intents as if they declared the CATEGORY_DEFAULT category. If you do not declare it in your intent filter, no implicit intents will resolve to your activity.
For more information about sending and receiving ACTION_SEND intents that perform social sharing behaviors, see the lesson about Receiving Simple Data from Other Apps.
Handle the Intent in Your Activity
In order to decide what action to take in your activity, you can read the Intent that was used to start it.
As your activity starts, call getIntent() to retrieve the Intent that started the activity. You can do so at any time during the lifecycle of the activity, but you should generally do so during early callbacks such as onCreate() or onStart().
For example:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
// Get the intent that started this activity
Intent intent = getIntent();
Uri data = intent.getData();
// Figure out what to do based on the intent type
if (intent.getType().indexOf("image/") != -1) {
// Handle intents with image data ...
} else if (intent.getType().equals("text/plain")) {
// Handle intents with text ...
}
}
Return a Result
If you want to return a result to the activity that invoked yours, simply call setResult() to specify the result code and result Intent. When your operation is done and the user should return to the original activity, call finish() to close (and destroy) your activity. For example:
// Create intent to deliver some kind of result data
Intent result = new Intent("com.example.RESULT_ACTION", Uri.parse("content://result_uri");
setResult(Activity.RESULT_OK, result);
finish();
You must always specify a result code with the result. Generally, it's either RESULT_OK or RESULT_CANCELED. You can then provide additional data with an Intent, as necessary.
Note: The result is set to RESULT_CANCELED by default. So, if the user presses the Back button before completing the action and before you set the result, the original activity receives the "canceled" result.
If you simply need to return an integer that indicates one of several result options, you can set the result code to any value higher than 0. If you use the result code to deliver an integer and you have no need to include the Intent, you can call setResult() and pass only a result code. For example:
setResult(RESULT_COLOR_RED);
finish();
In this case, there might be only a handful of possible results, so the result code is a locally defined integer (greater than 0). This works well when you're returning a result to an activity in your own app, because the activity that receives the result can reference the public constant to determine the value of the result code.
Note: There's no need to check whether your activity was started with startActivity() or startActivityForResult(). Simply call setResult() if the intent that started your activity might expect a result. If the originating activity had called startActivityForResult(), then the system delivers it the result you supply to setResult(); otherwise, the result is ignored.
The previous two lessons focused on one side of the story: starting another app's activity from your app. But if your app can perform an action that might be useful to another app, your app should be prepared to respond to action requests from other apps. For instance, if you build a social app that can share messages or photos with the user's friends, it's in your best interest to support the ACTION_SEND intent so users can initiate a "share" action from another app and launch your app to perform the action.
To allow other apps to start your activity, you need to add an
When your app is installed on a device, the system identifies your intent filters and adds the information to an internal catalog of intents supported by all installed apps. When an app calls startActivity() or startActivityForResult(), with an implicit intent, the system finds which activity (or activities) can respond to the intent.
Add an Intent Filter
In order to properly define which intents your activity can handle, each intent filter you add should be as specific as possible in terms of the type of action and data the activity accepts.
The system may send a given Intent to an activity if that activity has an intent filter fulfills the following criteria of the Intent object:
Action
A string naming the action to perform. Usually one of the platform-defined values such as ACTION_SEND or ACTION_VIEW.
Specify this in your intent filter with the
Data
A description of the data associated with the intent.
Specify this in your intent filter with the element. Using one or more attributes in this element, you can specify just the MIME type, just a URI prefix, just a URI scheme, or a combination of these and others that indicate the data type accepted.
Note: If you don't need to declare specifics about the data Uri (such as when your activity handles to other kind of "extra" data, instead of a URI), you should specify only the android:mimeType attribute to declare the type of data your activity handles, such as text/plain or image/jpeg.
Category
Provides an additional way to characterize the activity handling the intent, usually related to the user gesture or location from which it's started. There are several different categories supported by the system, but most are rarely used. However, all implicit intents are defined with CATEGORY_DEFAULT by default.
Specify this in your intent filter with the
In your intent filter, you can declare which criteria your activity accepts by declaring each of them with corresponding XML elements nested in the
For example, here's an activity with an intent filter that handles the ACTION_SEND intent when the data type is either text or an image:
Each incoming intent specifies only one action and one data type, but it's OK to declare multiple instances of the
Each incoming intent specifies only one action and one data type, but it's OK to declare multiple instances of the
If any two pairs of action and data are mutually exclusive in their behaviors, you should create separate intent filters to specify which actions are acceptable when paired with which data types.
For example, suppose your activity handles both text and images for both the ACTION_SEND and ACTION_SENDTO intents. In this case, you must define two separate intent filters for the two actions because a ACTION_SENDTO intent must use the data Uri to specify the recipient's address using the send or sendto URI scheme. For example:
Note: In order to receive implicit intents, you must include the CATEGORY_DEFAULT category in the intent filter. The methods startActivity() and startActivityForResult() treat all intents as if they declared the CATEGORY_DEFAULT category. If you do not declare it in your intent filter, no implicit intents will resolve to your activity.
For more information about sending and receiving ACTION_SEND intents that perform social sharing behaviors, see the lesson about Receiving Simple Data from Other Apps.
Handle the Intent in Your Activity
In order to decide what action to take in your activity, you can read the Intent that was used to start it.
As your activity starts, call getIntent() to retrieve the Intent that started the activity. You can do so at any time during the lifecycle of the activity, but you should generally do so during early callbacks such as onCreate() or onStart().
For example:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
// Get the intent that started this activity
Intent intent = getIntent();
Uri data = intent.getData();
// Figure out what to do based on the intent type
if (intent.getType().indexOf("image/") != -1) {
// Handle intents with image data ...
} else if (intent.getType().equals("text/plain")) {
// Handle intents with text ...
}
}
Return a Result
If you want to return a result to the activity that invoked yours, simply call setResult() to specify the result code and result Intent. When your operation is done and the user should return to the original activity, call finish() to close (and destroy) your activity. For example:
// Create intent to deliver some kind of result data
Intent result = new Intent("com.example.RESULT_ACTION", Uri.parse("content://result_uri");
setResult(Activity.RESULT_OK, result);
finish();
You must always specify a result code with the result. Generally, it's either RESULT_OK or RESULT_CANCELED. You can then provide additional data with an Intent, as necessary.
Note: The result is set to RESULT_CANCELED by default. So, if the user presses the Back button before completing the action and before you set the result, the original activity receives the "canceled" result.
If you simply need to return an integer that indicates one of several result options, you can set the result code to any value higher than 0. If you use the result code to deliver an integer and you have no need to include the Intent, you can call setResult() and pass only a result code. For example:
setResult(RESULT_COLOR_RED);
finish();
In this case, there might be only a handful of possible results, so the result code is a locally defined integer (greater than 0). This works well when you're returning a result to an activity in your own app, because the activity that receives the result can reference the public constant to determine the value of the result code.
Note: There's no need to check whether your activity was started with startActivity() or startActivityForResult(). Simply call setResult() if the intent that started your activity might expect a result. If the originating activity had called startActivityForResult(), then the system delivers it the result you supply to setResult(); otherwise, the result is ignored.
lessons 7.2 Getting a Result from an Activity
Getting a Result from an Activity
Starting another activity doesn't have to be one-way. You can also start another activity and receive a result back. To receive a result, call startActivityForResult() (instead of startActivity()).
For example, your app can start a camera app and receive the captured photo as a result. Or, you might start the People app in order for the user to select a contact and you'll receive the contact details as a result.
Of course, the activity that responds must be designed to return a result. When it does, it sends the result as another Intent object. Your activity receives it in the onActivityResult() callback.
Note: You can use explicit or implicit intents when you call startActivityForResult(). When starting one of your own activities to receive a result, you should use an explicit intent to ensure that you receive the expected result.
Start the Activity
There's nothing special about the Intent object you use when starting an activity for a result, but you do need to pass an additional integer argument to the startActivityForResult() method.
The integer argument is a "request code" that identifies your request. When you receive the result Intent, the callback provides the same request code so that your app can properly identify the result and determine how to handle it.
For example, here's how to start an activity that allows the user to pick a contact:
static final int PICK_CONTACT_REQUEST = 1; // The request code
...
private void pickContact() {
Intent pickContactIntent = new Intent(Intent.ACTION_PICK, Uri.parse("content://contacts"));
pickContactIntent.setType(Phone.CONTENT_TYPE); // Show user only contacts w/ phone numbers
startActivityForResult(pickContactIntent, PICK_CONTACT_REQUEST);
}
Receive the Result
When the user is done with the subsequent activity and returns, the system calls your activity's onActivityResult() method. This method includes three arguments:
The request code you passed to startActivityForResult().
A result code specified by the second activity. This is either RESULT_OK if the operation was successful or RESULT_CANCELED if the user backed out or the operation failed for some reason.
An Intent that carries the result data.
For example, here's how you can handle the result for the "pick a contact" intent:
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
// Check which request we're responding to
if (requestCode == PICK_CONTACT_REQUEST) {
// Make sure the request was successful
if (resultCode == RESULT_OK) {
// The user picked a contact.
// The Intent's data Uri identifies which contact was selected.
// Do something with the contact here (bigger example below)
}
}
}
In this example, the result Intent returned by Android's Contacts or People app provides a content Uri that identifies the contact the user selected.
In order to successfully handle the result, you must understand what the format of the result Intent will be. Doing so is easy when the activity returning a result is one of your own activities. Apps included with the Android platform offer their own APIs that you can count on for specific result data. For instance, the People app (Contacts app on some older versions) always returns a result with the content URI that identifies the selected contact, and the Camera app returns a Bitmap in the "data" extra (see the class about Capturing Photos).
Bonus: Read the contact data
The code above showing how to get a result from the People app doesn't go into details about how to actually read the data from the result, because it requires more advanced discussion about content providers. However, if you're curious, here's some more code that shows how to query the result data to get the phone number from the selected contact:
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
// Check which request it is that we're responding to
if (requestCode == PICK_CONTACT_REQUEST) {
// Make sure the request was successful
if (resultCode == RESULT_OK) {
// Get the URI that points to the selected contact
Uri contactUri = data.getData();
// We only need the NUMBER column, because there will be only one row in the result
String[] projection = {Phone.NUMBER};
// Perform the query on the contact to get the NUMBER column
// We don't need a selection or sort order (there's only one result for the given URI)
// CAUTION: The query() method should be called from a separate thread to avoid blocking
// your app's UI thread. (For simplicity of the sample, this code doesn't do that.)
// Consider using CursorLoader to perform the query.
Cursor cursor = getContentResolver()
.query(contactUri, projection, null, null, null);
cursor.moveToFirst();
// Retrieve the phone number from the NUMBER column
int column = cursor.getColumnIndex(Phone.NUMBER);
String number = cursor.getString(column);
// Do something with the phone number...
}
}
}
Note: Before Android 2.3 (API level 9), performing a query on the Contacts Provider (like the one shown above) requires that your app declare the READ_CONTACTS permission (see Security and Permissions). However, beginning with Android 2.3, the Contacts/People app grants your app a temporary permission to read from the Contacts Provider when it returns you a result. The temporary permission applies only to the specific contact requested, so you cannot query a contact other than the one specified by the intent's Uri, unless you do declare the READ_CONTACTS permission.
Starting another activity doesn't have to be one-way. You can also start another activity and receive a result back. To receive a result, call startActivityForResult() (instead of startActivity()).
For example, your app can start a camera app and receive the captured photo as a result. Or, you might start the People app in order for the user to select a contact and you'll receive the contact details as a result.
Of course, the activity that responds must be designed to return a result. When it does, it sends the result as another Intent object. Your activity receives it in the onActivityResult() callback.
Note: You can use explicit or implicit intents when you call startActivityForResult(). When starting one of your own activities to receive a result, you should use an explicit intent to ensure that you receive the expected result.
Start the Activity
There's nothing special about the Intent object you use when starting an activity for a result, but you do need to pass an additional integer argument to the startActivityForResult() method.
The integer argument is a "request code" that identifies your request. When you receive the result Intent, the callback provides the same request code so that your app can properly identify the result and determine how to handle it.
For example, here's how to start an activity that allows the user to pick a contact:
static final int PICK_CONTACT_REQUEST = 1; // The request code
...
private void pickContact() {
Intent pickContactIntent = new Intent(Intent.ACTION_PICK, Uri.parse("content://contacts"));
pickContactIntent.setType(Phone.CONTENT_TYPE); // Show user only contacts w/ phone numbers
startActivityForResult(pickContactIntent, PICK_CONTACT_REQUEST);
}
Receive the Result
When the user is done with the subsequent activity and returns, the system calls your activity's onActivityResult() method. This method includes three arguments:
The request code you passed to startActivityForResult().
A result code specified by the second activity. This is either RESULT_OK if the operation was successful or RESULT_CANCELED if the user backed out or the operation failed for some reason.
An Intent that carries the result data.
For example, here's how you can handle the result for the "pick a contact" intent:
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
// Check which request we're responding to
if (requestCode == PICK_CONTACT_REQUEST) {
// Make sure the request was successful
if (resultCode == RESULT_OK) {
// The user picked a contact.
// The Intent's data Uri identifies which contact was selected.
// Do something with the contact here (bigger example below)
}
}
}
In this example, the result Intent returned by Android's Contacts or People app provides a content Uri that identifies the contact the user selected.
In order to successfully handle the result, you must understand what the format of the result Intent will be. Doing so is easy when the activity returning a result is one of your own activities. Apps included with the Android platform offer their own APIs that you can count on for specific result data. For instance, the People app (Contacts app on some older versions) always returns a result with the content URI that identifies the selected contact, and the Camera app returns a Bitmap in the "data" extra (see the class about Capturing Photos).
Bonus: Read the contact data
The code above showing how to get a result from the People app doesn't go into details about how to actually read the data from the result, because it requires more advanced discussion about content providers. However, if you're curious, here's some more code that shows how to query the result data to get the phone number from the selected contact:
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
// Check which request it is that we're responding to
if (requestCode == PICK_CONTACT_REQUEST) {
// Make sure the request was successful
if (resultCode == RESULT_OK) {
// Get the URI that points to the selected contact
Uri contactUri = data.getData();
// We only need the NUMBER column, because there will be only one row in the result
String[] projection = {Phone.NUMBER};
// Perform the query on the contact to get the NUMBER column
// We don't need a selection or sort order (there's only one result for the given URI)
// CAUTION: The query() method should be called from a separate thread to avoid blocking
// your app's UI thread. (For simplicity of the sample, this code doesn't do that.)
// Consider using CursorLoader to perform the query.
Cursor cursor = getContentResolver()
.query(contactUri, projection, null, null, null);
cursor.moveToFirst();
// Retrieve the phone number from the NUMBER column
int column = cursor.getColumnIndex(Phone.NUMBER);
String number = cursor.getString(column);
// Do something with the phone number...
}
}
}
Note: Before Android 2.3 (API level 9), performing a query on the Contacts Provider (like the one shown above) requires that your app declare the READ_CONTACTS permission (see Security and Permissions). However, beginning with Android 2.3, the Contacts/People app grants your app a temporary permission to read from the Contacts Provider when it returns you a result. The temporary permission applies only to the specific contact requested, so you cannot query a contact other than the one specified by the intent's Uri, unless you do declare the READ_CONTACTS permission.
lessons 7.1 Sending the User to Another App
Sending the User to Another App
One of Android's most important features is an app's ability to send the user to another app based on an "action" it would like to perform. For example, if your app has the address of a business that you'd like to show on a map, you don't have to build an activity in your app that shows a map. Instead, you can create a request to view the address using an Intent. The Android system then starts an app that's able to show the address on a map.
As explained in the first class, Building Your First App, you must use intents to navigate between activities in your own app. You generally do so with an explicit intent, which defines the exact class name of the component you want to start. However, when you want to have a separate app perform an action, such as "view a map," you must use an implicit intent.
This lesson shows you how to create an implicit intent for a particular action, and how to use it to start an activity that performs the action in another app.
Build an Implicit Intent
Implicit intents do not declare the class name of the component to start, but instead declare an action to perform. The action specifies the thing you want to do, such as view, edit, send, or get something. Intents often also include data associated with the action, such as the address you want to view, or the email message you want to send. Depending on the intent you want to create, the data might be a Uri, one of several other data types, or the intent might not need data at all.
If your data is a Uri, there's a simple Intent() constructor you can use define the action and data.
For example, here's how to create an intent to initiate a phone call using the Uri data to specify the telephone number:
Uri number = Uri parse("tel:5551234");
Intent callIntent = new Intent(Intent.ACTION_DIAL, number);
When your app invokes this intent by calling startActivity(), the Phone app initiates a call to the given phone number.
Here are a couple other intents and their action and Uri data pairs:
View a map:
// Map point based on address
Uri location = Uri.parse("geo:0,0?q=1600+Amphitheatre+Parkway,+Mountain+View,+California");
// Or map point based on latitude/longitude
// Uri location = Uri.parse("geo:37.422219,-122.08364?z=14"); // z param is zoom level
Intent mapIntent = new Intent(Intent.ACTION_VIEW, location);
View a web page:
Uri webpage = Uri.parse("http://www.android.com");
Intent webIntent = new Intent(Intent.ACTION_VIEW, webpage);
Other kinds of implicit intents require "extra" data that provide different data types, such as a string. You can add one or more pieces of extra data using the various putExtra() methods.
By default, the system determines the appropriate MIME type required by an intent based on the Uri data that's included. If you don't include a Uri in the intent, you should usually use setType() to specify the type of data associated with the intent. Setting the MIME type further specifies which kinds of activities should receive the intent.
Here are some more intents that add extra data to specify the desired action:
Send an email with an attachment:
Intent emailIntent = new Intent(Intent.ACTION_SEND);
// The intent does not have a URI, so declare the "text/plain" MIME type
emailIntent.setType(HTTP.PLAIN_TEXT_TYPE);
emailIntent.putExtra(Intent.EXTRA_EMAIL, new String[] {"jon@example.com"}); // recipients
emailIntent.putExtra(Intent.EXTRA_SUBJECT, "Email subject");
emailIntent.putExtra(Intent.EXTRA_TEXT, "Email message text");
emailIntent.putExtra(Intent.EXTRA_STREAM, Uri.parse("content://path/to/email/attachment"));
// You can also attach multiple items by passing an ArrayList of Uris
Create a calendar event:
Intent calendarIntent = new Intent(Intent.ACTION_INSERT, Events.CONTENT_URI);
Calendar beginTime = Calendar.getInstance().set(2012, 0, 19, 7, 30);
Calendar endTime = Calendar.getInstance().set(2012, 0, 19, 10, 30);
calendarIntent.putExtra(CalendarContract.EXTRA_EVENT_BEGIN_TIME, beginTime.getTimeInMillis());
calendarIntent.putExtra(CalendarContract.EXTRA_EVENT_END_TIME, endTime.getTimeInMillis());
calendarIntent.putExtra(Events.TITLE, "Ninja class");
calendarIntent.putExtra(Events.EVENT_LOCATION, "Secret dojo");
Note: This intent for a calendar event is supported only with API level 14 and higher.
Note: It's important that you define your Intent to be as specific as possible. For example, if you want to display an image using the ACTION_VIEW intent, you should specify a MIME type of image/*. This prevents apps that can "view" other types of data (like a map app) from being triggered by the intent.
Verify There is an App to Receive the Intent
Although the Android platform guarantees that certain intents will resolve to one of the built-in apps (such as the Phone, Email, or Calendar app), you should always include a verification step before invoking an intent.
Caution: If you invoke an intent and there is no app available on the device that can handle the intent, your app will crash.
To verify there is an activity available that can respond to the intent, call queryIntentActivities() to get a list of activities capable of handling your Intent. If the returned List is not empty, you can safely use the intent. For example:
PackageManager packageManager = getPackageManager();
List activities = packageManager.queryIntentActivities(intent, 0);
boolean isIntentSafe = activities.size() > 0;
If isIntentSafe is true, then at least one app will respond to the intent. If it is false, then there aren't any apps to handle the intent.
Note: You should perform this check when your activity first starts in case you need to disable the feature that uses the intent before the user attempts to use it. If you know of a specific app that can handle the intent, you can also provide a link for the user to download the app (see how to link to your product on Google Play).
Start an Activity with the Intent
Figure 1. Example of the selection dialog that appears when more than one app can handle an intent.
Once you have created your Intent and set the extra info, call startActivity() to send it to the system. If the system identifies more than one activity that can handle the intent, it displays a dialog for the user to select which app to use, as shown in figure 1. If there is only one activity that handles the intent, the system immediately starts it.
startActivity(intent);
Here's a complete example that shows how to create an intent to view a map, verify that an app exists to handle the intent, then start it:
// Build the intent
Uri location = Uri.parse("geo:0,0?q=1600+Amphitheatre+Parkway,+Mountain+View,+California");
Intent mapIntent = new Intent(Intent.ACTION_VIEW, location);
// Verify it resolves
PackageManager packageManager = getPackageManager();
List activities = packageManager.queryIntentActivities(mapIntent, 0);
boolean isIntentSafe = activities.size() > 0;
// Start an activity if it's safe
if (isIntentSafe) {
startActivity(mapIntent);
}
Show an App Chooser
Figure 2. A chooser dialog.
Notice that when you start an activity by passing your Intent to startActivity() and there is more than one app that responds to the intent, the user can select which app to use by default (by selecting a checkbox at the bottom of the dialog; see figure 1). This is nice when performing an action for which the user generally wants to use the same app every time, such as when opening a web page (users likely use just one web browser) or taking a photo (users likely prefer one camera).
However, if the action to be performed could be handled by multiple apps and the user might prefer a different app each time—such as a "share" action, for which users might have several apps through which they might share an item—you should explicitly show a chooser dialog as shown in figure 2. The chooser dialog forces the user to select which app to use for the action every time (the user cannot select a default app for the action).
To show the chooser, create an Intent using createChooser() and pass it to startActivity(). For example:
Intent intent = new Intent(Intent.ACTION_SEND);
...
// Always use string resources for UI text.
// This says something like "Share this photo with"
String title = getResources().getString(R.string.chooser_title);
// Create intent to show chooser
Intent chooser = Intent.createChooser(intent, title);
// Verify the intent will resolve to at least one activity
if (intent.resolveActivity(getPackageManager()) != null) {
startActivity(chooser);
}
This displays a dialog with a list of apps that respond to the intent passed to the createChooser() method and uses the supplied text as the dialog title.
One of Android's most important features is an app's ability to send the user to another app based on an "action" it would like to perform. For example, if your app has the address of a business that you'd like to show on a map, you don't have to build an activity in your app that shows a map. Instead, you can create a request to view the address using an Intent. The Android system then starts an app that's able to show the address on a map.
As explained in the first class, Building Your First App, you must use intents to navigate between activities in your own app. You generally do so with an explicit intent, which defines the exact class name of the component you want to start. However, when you want to have a separate app perform an action, such as "view a map," you must use an implicit intent.
This lesson shows you how to create an implicit intent for a particular action, and how to use it to start an activity that performs the action in another app.
Build an Implicit Intent
Implicit intents do not declare the class name of the component to start, but instead declare an action to perform. The action specifies the thing you want to do, such as view, edit, send, or get something. Intents often also include data associated with the action, such as the address you want to view, or the email message you want to send. Depending on the intent you want to create, the data might be a Uri, one of several other data types, or the intent might not need data at all.
If your data is a Uri, there's a simple Intent() constructor you can use define the action and data.
For example, here's how to create an intent to initiate a phone call using the Uri data to specify the telephone number:
Uri number = Uri parse("tel:5551234");
Intent callIntent = new Intent(Intent.ACTION_DIAL, number);
When your app invokes this intent by calling startActivity(), the Phone app initiates a call to the given phone number.
Here are a couple other intents and their action and Uri data pairs:
View a map:
// Map point based on address
Uri location = Uri.parse("geo:0,0?q=1600+Amphitheatre+Parkway,+Mountain+View,+California");
// Or map point based on latitude/longitude
// Uri location = Uri.parse("geo:37.422219,-122.08364?z=14"); // z param is zoom level
Intent mapIntent = new Intent(Intent.ACTION_VIEW, location);
View a web page:
Uri webpage = Uri.parse("http://www.android.com");
Intent webIntent = new Intent(Intent.ACTION_VIEW, webpage);
Other kinds of implicit intents require "extra" data that provide different data types, such as a string. You can add one or more pieces of extra data using the various putExtra() methods.
By default, the system determines the appropriate MIME type required by an intent based on the Uri data that's included. If you don't include a Uri in the intent, you should usually use setType() to specify the type of data associated with the intent. Setting the MIME type further specifies which kinds of activities should receive the intent.
Here are some more intents that add extra data to specify the desired action:
Send an email with an attachment:
Intent emailIntent = new Intent(Intent.ACTION_SEND);
// The intent does not have a URI, so declare the "text/plain" MIME type
emailIntent.setType(HTTP.PLAIN_TEXT_TYPE);
emailIntent.putExtra(Intent.EXTRA_EMAIL, new String[] {"jon@example.com"}); // recipients
emailIntent.putExtra(Intent.EXTRA_SUBJECT, "Email subject");
emailIntent.putExtra(Intent.EXTRA_TEXT, "Email message text");
emailIntent.putExtra(Intent.EXTRA_STREAM, Uri.parse("content://path/to/email/attachment"));
// You can also attach multiple items by passing an ArrayList of Uris
Create a calendar event:
Intent calendarIntent = new Intent(Intent.ACTION_INSERT, Events.CONTENT_URI);
Calendar beginTime = Calendar.getInstance().set(2012, 0, 19, 7, 30);
Calendar endTime = Calendar.getInstance().set(2012, 0, 19, 10, 30);
calendarIntent.putExtra(CalendarContract.EXTRA_EVENT_BEGIN_TIME, beginTime.getTimeInMillis());
calendarIntent.putExtra(CalendarContract.EXTRA_EVENT_END_TIME, endTime.getTimeInMillis());
calendarIntent.putExtra(Events.TITLE, "Ninja class");
calendarIntent.putExtra(Events.EVENT_LOCATION, "Secret dojo");
Note: This intent for a calendar event is supported only with API level 14 and higher.
Note: It's important that you define your Intent to be as specific as possible. For example, if you want to display an image using the ACTION_VIEW intent, you should specify a MIME type of image/*. This prevents apps that can "view" other types of data (like a map app) from being triggered by the intent.
Verify There is an App to Receive the Intent
Although the Android platform guarantees that certain intents will resolve to one of the built-in apps (such as the Phone, Email, or Calendar app), you should always include a verification step before invoking an intent.
Caution: If you invoke an intent and there is no app available on the device that can handle the intent, your app will crash.
To verify there is an activity available that can respond to the intent, call queryIntentActivities() to get a list of activities capable of handling your Intent. If the returned List is not empty, you can safely use the intent. For example:
PackageManager packageManager = getPackageManager();
List
boolean isIntentSafe = activities.size() > 0;
If isIntentSafe is true, then at least one app will respond to the intent. If it is false, then there aren't any apps to handle the intent.
Note: You should perform this check when your activity first starts in case you need to disable the feature that uses the intent before the user attempts to use it. If you know of a specific app that can handle the intent, you can also provide a link for the user to download the app (see how to link to your product on Google Play).
Start an Activity with the Intent
Figure 1. Example of the selection dialog that appears when more than one app can handle an intent.
Once you have created your Intent and set the extra info, call startActivity() to send it to the system. If the system identifies more than one activity that can handle the intent, it displays a dialog for the user to select which app to use, as shown in figure 1. If there is only one activity that handles the intent, the system immediately starts it.
startActivity(intent);
Here's a complete example that shows how to create an intent to view a map, verify that an app exists to handle the intent, then start it:
// Build the intent
Uri location = Uri.parse("geo:0,0?q=1600+Amphitheatre+Parkway,+Mountain+View,+California");
Intent mapIntent = new Intent(Intent.ACTION_VIEW, location);
// Verify it resolves
PackageManager packageManager = getPackageManager();
List
boolean isIntentSafe = activities.size() > 0;
// Start an activity if it's safe
if (isIntentSafe) {
startActivity(mapIntent);
}
Show an App Chooser
Figure 2. A chooser dialog.
Notice that when you start an activity by passing your Intent to startActivity() and there is more than one app that responds to the intent, the user can select which app to use by default (by selecting a checkbox at the bottom of the dialog; see figure 1). This is nice when performing an action for which the user generally wants to use the same app every time, such as when opening a web page (users likely use just one web browser) or taking a photo (users likely prefer one camera).
However, if the action to be performed could be handled by multiple apps and the user might prefer a different app each time—such as a "share" action, for which users might have several apps through which they might share an item—you should explicitly show a chooser dialog as shown in figure 2. The chooser dialog forces the user to select which app to use for the action every time (the user cannot select a default app for the action).
To show the chooser, create an Intent using createChooser() and pass it to startActivity(). For example:
Intent intent = new Intent(Intent.ACTION_SEND);
...
// Always use string resources for UI text.
// This says something like "Share this photo with"
String title = getResources().getString(R.string.chooser_title);
// Create intent to show chooser
Intent chooser = Intent.createChooser(intent, title);
// Verify the intent will resolve to at least one activity
if (intent.resolveActivity(getPackageManager()) != null) {
startActivity(chooser);
}
This displays a dialog with a list of apps that respond to the intent passed to the createChooser() method and uses the supplied text as the dialog title.
Sunday, 29 December 2013
lessons 7.0 Interacting with Other Apps
Interacting with Other Apps
Basic understanding of the Activity lifecycle (see Managing the Activity Lifecycle)
YOU SHOULD ALSO READ
Sharing Simple Data
Sharing Files
Integrating Application with Intents (blog post)
Intents and Intent Filters
An Android app typically has several activities. Each activity displays a user interface that allows the user to perform a specific task (such as view a map or take a photo). To take the user from one activity to another, your app must use an Intent to define your app's "intent" to do something. When you pass an Intent to the system with a method such as startActivity(), the system uses the Intent to identify and start the appropriate app component. Using intents even allows your app to start an activity that is contained in a separate app.
An Intent can be explicit in order to start a specific component (a specific Activity instance) or implicit in order to start any component that can handle the intended action (such as "capture a photo").
This class shows you how to use an Intent to perform some basic interactions with other apps, such as start another app, receive a result from that app, and make your app able to respond to intents from other apps.
Basic understanding of the Activity lifecycle (see Managing the Activity Lifecycle)
YOU SHOULD ALSO READ
Sharing Simple Data
Sharing Files
Integrating Application with Intents (blog post)
Intents and Intent Filters
An Android app typically has several activities. Each activity displays a user interface that allows the user to perform a specific task (such as view a map or take a photo). To take the user from one activity to another, your app must use an Intent to define your app's "intent" to do something. When you pass an Intent to the system with a method such as startActivity(), the system uses the Intent to identify and start the appropriate app component. Using intents even allows your app to start an activity that is contained in a separate app.
An Intent can be explicit in order to start a specific component (a specific Activity instance) or implicit in order to start any component that can handle the intended action (such as "capture a photo").
This class shows you how to use an Intent to perform some basic interactions with other apps, such as start another app, receive a result from that app, and make your app able to respond to intents from other apps.
lessons 6.3 Saving Data in SQL Databases
Saving Data in SQL Databases
THIS LESSON TEACHES YOU TO
Define a Schema and Contract
Create a Database Using a SQL Helper
Put Information into a Database
Read Information from a Database
Delete Information from a Database
Update a Database
YOU SHOULD ALSO READ
Using Databases
Saving data to a database is ideal for repeating or structured data, such as contact information. This class assumes that you are familiar with SQL databases in general and helps you get started with SQLite databases on Android. The APIs you'll need to use a database on Android are available in the android.database.sqlite package.
Define a Schema and Contract
One of the main principles of SQL databases is the schema: a formal declaration of how the database is organized. The schema is reflected in the SQL statements that you use to create your database. You may find it helpful to create a companion class, known as a contract class, which explicitly specifies the layout of your schema in a systematic and self-documenting way.
A contract class is a container for constants that define names for URIs, tables, and columns. The contract class allows you to use the same constants across all the other classes in the same package. This lets you change a column name in one place and have it propagate throughout your code.
A good way to organize a contract class is to put definitions that are global to your whole database in the root level of the class. Then create an inner class for each table that enumerates its columns.
Note: By implementing the BaseColumns interface, your inner class can inherit a primary key field called _ID that some Android classes such as cursor adaptors will expect it to have. It's not required, but this can help your database work harmoniously with the Android framework.
For example, this snippet defines the table name and column names for a single table:
public final class FeedReaderContract {
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
public FeedReaderContract() {}
/* Inner class that defines the table contents */
public static abstract class FeedEntry implements BaseColumns {
public static final String TABLE_NAME = "entry";
public static final String COLUMN_NAME_ENTRY_ID = "entryid";
public static final String COLUMN_NAME_TITLE = "title";
public static final String COLUMN_NAME_SUBTITLE = "subtitle";
...
}
}
Create a Database Using a SQL Helper
Once you have defined how your database looks, you should implement methods that create and maintain the database and tables. Here are some typical statements that create and delete a table:
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
private static final String SQL_CREATE_ENTRIES =
"CREATE TABLE " + FeedEntry.TABLE_NAME + " (" +
FeedEntry._ID + " INTEGER PRIMARY KEY," +
FeedEntry.COLUMN_NAME_ENTRY_ID + TEXT_TYPE + COMMA_SEP +
FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP +
... // Any other options for the CREATE command
" )";
private static final String SQL_DELETE_ENTRIES =
"DROP TABLE IF EXISTS " + FeedEntry.TABLE_NAME;
Just like files that you save on the device's internal storage, Android stores your database in private disk space that's associated application. Your data is secure, because by default this area is not accessible to other applications.
A useful set of APIs is available in the SQLiteOpenHelper class. When you use this class to obtain references to your database, the system performs the potentially long-running operations of creating and updating the database only when needed and not during app startup. All you need to do is call getWritableDatabase() or getReadableDatabase().
Note: Because they can be long-running, be sure that you call getWritableDatabase() or getReadableDatabase() in a background thread, such as with AsyncTask or IntentService.
To use SQLiteOpenHelper, create a subclass that overrides the onCreate(), onUpgrade() and onOp
THIS LESSON TEACHES YOU TO
Define a Schema and Contract
Create a Database Using a SQL Helper
Put Information into a Database
Read Information from a Database
Delete Information from a Database
Update a Database
YOU SHOULD ALSO READ
Using Databases
Saving data to a database is ideal for repeating or structured data, such as contact information. This class assumes that you are familiar with SQL databases in general and helps you get started with SQLite databases on Android. The APIs you'll need to use a database on Android are available in the android.database.sqlite package.
Define a Schema and Contract
One of the main principles of SQL databases is the schema: a formal declaration of how the database is organized. The schema is reflected in the SQL statements that you use to create your database. You may find it helpful to create a companion class, known as a contract class, which explicitly specifies the layout of your schema in a systematic and self-documenting way.
A contract class is a container for constants that define names for URIs, tables, and columns. The contract class allows you to use the same constants across all the other classes in the same package. This lets you change a column name in one place and have it propagate throughout your code.
A good way to organize a contract class is to put definitions that are global to your whole database in the root level of the class. Then create an inner class for each table that enumerates its columns.
Note: By implementing the BaseColumns interface, your inner class can inherit a primary key field called _ID that some Android classes such as cursor adaptors will expect it to have. It's not required, but this can help your database work harmoniously with the Android framework.
For example, this snippet defines the table name and column names for a single table:
public final class FeedReaderContract {
// To prevent someone from accidentally instantiating the contract class,
// give it an empty constructor.
public FeedReaderContract() {}
/* Inner class that defines the table contents */
public static abstract class FeedEntry implements BaseColumns {
public static final String TABLE_NAME = "entry";
public static final String COLUMN_NAME_ENTRY_ID = "entryid";
public static final String COLUMN_NAME_TITLE = "title";
public static final String COLUMN_NAME_SUBTITLE = "subtitle";
...
}
}
Create a Database Using a SQL Helper
Once you have defined how your database looks, you should implement methods that create and maintain the database and tables. Here are some typical statements that create and delete a table:
private static final String TEXT_TYPE = " TEXT";
private static final String COMMA_SEP = ",";
private static final String SQL_CREATE_ENTRIES =
"CREATE TABLE " + FeedEntry.TABLE_NAME + " (" +
FeedEntry._ID + " INTEGER PRIMARY KEY," +
FeedEntry.COLUMN_NAME_ENTRY_ID + TEXT_TYPE + COMMA_SEP +
FeedEntry.COLUMN_NAME_TITLE + TEXT_TYPE + COMMA_SEP +
... // Any other options for the CREATE command
" )";
private static final String SQL_DELETE_ENTRIES =
"DROP TABLE IF EXISTS " + FeedEntry.TABLE_NAME;
Just like files that you save on the device's internal storage, Android stores your database in private disk space that's associated application. Your data is secure, because by default this area is not accessible to other applications.
A useful set of APIs is available in the SQLiteOpenHelper class. When you use this class to obtain references to your database, the system performs the potentially long-running operations of creating and updating the database only when needed and not during app startup. All you need to do is call getWritableDatabase() or getReadableDatabase().
Note: Because they can be long-running, be sure that you call getWritableDatabase() or getReadableDatabase() in a background thread, such as with AsyncTask or IntentService.
To use SQLiteOpenHelper, create a subclass that overrides the onCreate(), onUpgrade() and onOp
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