Java Tutorial - Java While

The while loop repeats a statement or block while its controlling condition is true.

Java while Loop

Here is its general form:

while(condition) { 
 // body of loop 
}

The condition can be any Boolean expression.
The body of the loop will be executed as long as the conditional condition is true.
The curly braces are unnecessary if only a single statement is being repeated.

Here is a while loop that counts down from 10, printing exactly ten lines of "tick":

 
public class Main {
 public static void main(String args[]) {
 int n = 10;
 while (n > 0) {
 System.out.println("n:" + n);
 n--;
 }
 }
}

When you run this program, you will get the following result:

Example

The following code shows how to use the while loop to calculate sum.

public class Main {
 public static void main(String[] args) {
 int limit = 20;
 int sum = 0;/*fromwww.java2s.com*/
 int i = 1;
 while (i <= limit) {
 sum += i++;
 }
 System.out.println("sum = " + sum);
 }
}

The code above generates the following result.

Example 2

The body of the while loop will not execute if the condition is false. For example, in the following fragment, the call to println() is never executed:

 
public class Main {
 public static void main(String[] argv) {
 int a = 10, b = 20;
 while (a > b) {
 System.out.println("This will not be displayed");
 }
 System.out.println("You are here");
 }
}

The output:

Example 3

The body of the while can be empty. For example, consider the following program:

 
public class Main {
 public static void main(String args[]) {
 int i, j;//fromwww.java2s.com
 i = 10;
 j = 20;
 // find midpoint between i and j
 while (++i < --j)
 ;
 System.out.println("Midpoint is " + i);
 }
}

The while loop in the code above has no loop body and i and j are calculated in the while loop condition statement. It generates the following output:

Java do while loop

To execute the body of a while loop at least once, you can use the do-while loop.

The syntax for Java do while loop is:

do { 
 // body of loop 
} while (condition);

Here is an example to show how to use a do-while loop.

 
public class Main {
 public static void main(String args[]) {
 int n = 10;
 do {
 System.out.println("n:" + n);
 n--;
 } while (n > 0);
 }
}

The output:

The loop in the preceding program can be written as follows:

public class Main {
 public static void main(String args[]) {
 int n = 10;
 do {
 System.out.println("n:" + n);
 } while (--n > 0);
 }
}

The output is identical the result above:

Example 4

The following program implements a very simple help system with do-while loop and switch statement.

 
public class Main {
 public static void main(String args[]) throws java.io.IOException {
 char choice;//www.java2s.com
 do {
 System.out.println("Help on:");
 System.out.println(" 1. A");
 System.out.println(" 2. B");
 System.out.println(" 3. C");
 System.out.println(" 4. D");
 System.out.println(" 5. E");
 System.out.println("Choose one:");
 choice = (char) System.in.read();
 } while (choice < '1' || choice > '5');
 System.out.println("\n");
 switch (choice) {
 case '1':
 System.out.println("A");
 break;
 case '2':
 System.out.println("B");
 break;
 case '3':
 System.out.println("C");
 break;
 case '4':
 System.out.println("D");
 break;
 case '5':
 System.out.println("E");
 break;
 }
 }
}

Here is a sample run produced by this program: