Do While Loop in Java

The do-while loop is an exit-controlled loop — the body runs at least once before the condition is checked. It is perfect for menu programs, retry prompts and any code that must execute at least one time regardless of the condition.

• Syntax: do { body } while (condition);
• A semicolon (;) is required after the closing while().
• The body executes before the condition is checked.
• Useful when at least one iteration is needed.

Let's start with a hands-on example. The program below shows the core idea behind Do While Loop in just a few lines of Java. Read it line by line and observe how Java executes each statement in order.

class Main
{
    public static void main(String args[])
    {
        int i = 1;

        do {
            System.out.println(i);
            i++;
        } while (i <= 5);
    }
}

The following examples progress from simple to more practical patterns. Try each in your IDE, change the inputs and observe the output. This is the fastest way to internalize the concept.

class Main
{
    public static void main(String args[])
    {
        int n = 10, sum = 0, i = 1;

        do {
            sum += i;
            i++;
        } while (i <= n);

        System.out.println("Sum = " + sum);
    }
}

import java.util.Scanner;

class Main
{
    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);
        int choice;

        do {
            System.out.println("1.Hello  2.Bye  3.Exit");
            choice = sc.nextInt();

            if (choice == 1) System.out.println("Hello!");
            else if (choice == 2) System.out.println("Bye!");

        } while (choice != 3);
    }
}

class Main
{
    public static void main(String args[])
    {
        int x = 100;

        do {
            System.out.println("Executed once");
        } while (x < 10);
    }
}

class Main
{
    public static void main(String args[])
    {
        int n = 6789, rev = 0;

        do {
            rev = rev * 10 + n % 10;
            n /= 10;
        } while (n != 0);

        System.out.println("Reverse = " + rev);
    }
}

• Do not forget the semicolon after while(condition).
• Use do-while when you want the body to run at least once.
• Do-while is slightly less common than while, but very expressive for menus.
• Initialize variables before the loop, not inside the condition.
• Combine do-while with try-catch for input validation loops.

Reading is not enough — practice solidifies knowledge. Try every question below in your own editor before peeking at any solution.

• Q1. Write a do-while loop that prints all even numbers from 2 to 20.
• Q2. Build a menu program with 4 options using do-while.
• Q3. Sum digits of a number using do-while.
• Q4. Print factors of a number using do-while.
• Q5. Repeatedly ask the user for a positive number until they enter one.

These are the most common questions asked in Java interviews on this topic. Memorize the concept, not just the answer — interviewers often follow up with edge cases.

Even experienced developers slip on the same pitfalls. Watch out for these classic mistakes while working with Do While Loop in Java:

• Forgetting the semicolon ; at the end of statements while using Do While Loop.
• Mixing up similar method names — read Java docs before using new APIs related to Do While Loop.
• Ignoring compiler warnings — they often hint at bugs that will appear later in production.
• Hard-coding values that should come from configuration files or environment variables.
• Not handling edge cases: empty inputs, very large inputs, negative numbers and null references.
• Skipping unit tests — small tests prevent big regressions, especially around control flow.