Java do while Loop with Examples

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In this chapter, we will discuss how to use a do-while loop with examples. 

The do-while loop is similar to the while loop, however, there is a difference between them: In a while loop, a condition is evaluated before the execution of the loop’s body but in a do-while loop, a condition is evaluated after the execution of the loop’s body.

The do-while loop always executes its body at least once, because its conditional expression is at the bottom of the loop. 

Table of contents

1. do-while Loop Syntax
2. How do-while Loop Work?
3. Simple do-while Loop Example
4. do-while Loop with Menu Selection Example

1. do-while Loop Syntax

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

Each iteration of the do-while loop first executes the body of the loop and then evaluates the conditional expression. If this expression is true, the loop will repeat. Otherwise, the loop terminates. As with all of Java’s loops, a condition must be a Boolean expression.

2. How do-while Loop Works?

First, the statements inside the loop execute and then the condition gets evaluated, if the condition returns true then the control gets transferred to the “do” else it jumps to the next statement after do-while.

3. Simple do-while Loop Example

package net.javaguides.corejava.controlstatements.loops;

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

Output:

tick 10
tick 9
tick 8
tick 7
tick 6
tick 5
tick 4
tick 3
tick 2
tick 1

3. do-while Loop with Menu Selection Example

The do-while loop is especially useful when you process a menu selection because you will usually want the body of a menu loop to execute at least once.

Consider the following program, which implements a very simple help system for Java’s selection and iteration statements:

package net.javaguides.corejava.controlstatements.loops;

public class DoWhileMenuExample {
    public static void main(String args[]) throws java.io.IOException {
        char choice;
        do {
            System.out.println("Help on: ");
            System.out.println(" 1. if");
            System.out.println(" 2. switch");
            System.out.println(" 3. while");
            System.out.println(" 4. do-while");
            System.out.println(" 5. for\n");
            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("The if:\n");
                System.out.println("if(condition) statement;");
                System.out.println("else statement;");
                break;
            case '2':
                System.out.println("The switch:\n");
                System.out.println("switch(expression) {");
                System.out.println(" case constant:");
                System.out.println(" statement sequence");
                System.out.println(" break;");
                System.out.println(" //...");
                System.out.println("}");
                break;
            case '3':
                System.out.println("The while:\n");
                System.out.println("while(condition) statement;");
                break;
            case '4':
                System.out.println("The do-while:\n");
                System.out.println("do {");
                System.out.println(" statement;");
                System.out.println("} while (condition);");
                break;
            case '5':
                System.out.println("The for:\n");
                System.out.print("for(init; condition; iteration)");
                System.out.println(" statement;");
                break;
        }
    }
}

Here is a sample run produced by this program:

Help on:
1. if
2. switch
3. while
4. do-while
5. for
Choose one:
4
The do-while:
do {
statement;
} while (condition);

In the program, the do-while loop is used to verify that the user has entered a valid choice. If not, then the user is re-prompted. Since the menu must be displayed at least once, the do-while is the perfect loop to accomplish this.

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