Java do-while Loop

In this article, you will learn about the do-while loop and how to create do-while loop in Java programming.

Loops in Java are used when we want to execute a block of statements repeatedly until a specific condition is met(condition is false).

Java do-while Loop

Java do-while loop is an exit-control loop, which means unlike while loop, a do-while loop check for the condition after executing the statements or the loop body.

The do-while loop is similar to while loop with one main difference. The body of the do-while loop is executed at least once before the test-expression is checked.

Note: the do-while loop will execute the statements at least once before any condition is checked.

Syntax

do {
        //body of the loop
        //statements to be executed
        }
     //test expression is a boolean expression
        while (test_expression);

Flowchart of do-while loop

How do-while loop works?

When the Control falls into the do-while loop, it first executes the body of the loop inside the do block and then it checks the test_expression.

If the test expression is evaluated to true,

The statements inside the do block(body of the loop) get executed.
Then, the test expression is evaluated again and this process goes on until the test expression is evaluated to false.

If the test expression is evaluated to false,

do-while loop gets terminated and control goes out of the do-while loop.

Example 1

public class DoWhileLoopDemo {

    public static void main(String[] args) {

        int i = 1;

        do {
            System.out.println("value of i: " + i);

            //increment value of i by 1
            i++;
        }
        //exit when i becomes greater than 3
        while (i < 4);
    }
}

Output

value of i: 1
value of i: 2
value of i: 3

Example explained

In the above example,

1. i is initialized with value 1.

2. Execute the loop body inside do block.

“value of i: 1” gets printed.

Updation is done. Now i = 2.

3. Condition 2 < 4 is checked, yields true.

4. Execute the loop body inside do block.

“value of i: 2” gets printed.
Updation is done. Now i = 3.

5. Condition 3< 4 is checked, yields true.

6. Execute the loop body inside do block.

“value of i: 3” gets printed.
Updation is done. Now i = 4.

7. Condition 4< 4 is checked, yields false.

8. Exit from the do-while loop.

Example 2

The below program will print the sum of natural numbers from 1 to 10.

public class DoWhileLoopDemo {

    public static void main(String[] args) {

        int i = 1, sum = 0;

        do {

            sum = sum + i;

            //Increment the value of i for
            //next iteration
            i++;
        }

        // Exit when i becomes greater than 10
        while (i <= 10);

        System.out.println("Sum of numbers from 1 to 10: " + sum);
    }
}

Output

Sum of numbers from 1 to 10: 55

Java do-while Loop