Java - do…while Loop

Java 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.

The do-while loop is an exit control loop, where the condition is checked after executing the loop’s body.

Syntax of do while Loop

Following is the syntax of a do…while loop −

do {
   // Statements
}while(Boolean_expression);

Execution Process of a do while Loop

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 control jumps back up to do statement, and the statements in the loop execute again. This process repeats until the Boolean expression is false.

Flow Diagram

The following diagram shows the flow diagram (execution process) of a do while loop in Java -

do while Loop Examples

Example 1: Printing Numbers in a Range Using do while

In this example, we’re showing the use of a while loop to print numbers starting from 10 to 19. Here we’ve initialized an int variable x with a value of 10. Then in do while loop, we’re checking x as less than 20 after do while loop body. In do while loop body we’re printing the value of x and incrementing the value of x by 1. While loop will run until x becomes 20. Once x is 20, loop will stop execution and program exits.

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 );
   }
}

Output

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

Example 2: Printing Elements of an Array Using do while

In this example, we’re showing the use of a do while loop to print contents of an array. Here we’re creating an array of integers as numbers and initialized it some values. We’ve created a variable named index to represent index of the array while iterating it. In do while loop we’re checking the index to be less than size of the array after loop body and printed the element of the array using index notation. Within loop body index variable is incremented by 1 and loop continues till index becomes the sie of the array and loop exits.

public class Test {

   public static void main(String args[]) {
      int [] numbers = {10, 20, 30, 40, 50};
      int index = 0;

      do {
         System.out.print("value of item : " + numbers[index] );
         index++;
         System.out.print("\n");
      } while( index < 5 );
   }
}

Output

value of item : 10
value of item : 20
value of item : 30
value of item : 40
value of item : 50

do while Infinite Loop in Java

An infinite loop can also be implemented by writing “true” as the conditional statement using do do-while loop statement in Java.

Example: Implementing an infinite do while Loop

In this example, we’re showing the infinite loop using while loop. It will keep printing the numbers until you press ctrl+c to terminate the program.

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( true );
   }
}

Output

value of item : 10
value of item : 20
value of item : 30
value of item : 40
value of item : 50
...
ctrl+c