Java Do-While Loop

A while loop checks its condition first, so it might never run. But some tasks must happen at least once before you can even decide whether to repeat: showing a menu, asking for a PIN, taking a first guess. The Java do while loop is built for this. In this lesson, you'll learn how the do while loop works, how it differs from while, and two patterns you'll use constantly: menus and input validation.

What is a do while loop in Java?

A do while loop runs its body, then checks the condition. If the condition is true, it goes back and runs the body again. Because the check comes at the end, the body is guaranteed to run at least once.

do {
    // body: always runs at least once
} while (condition);   // note the semicolon

public class ConnectRetry {
    public static void main(String[] args) {
        int attempt = 0;

        do {
            attempt++;
            System.out.println("Connecting to server... attempt " + attempt);
        } while (attempt < 3);

        System.out.println("Gave up after " + attempt + " attempts.");
    }
}

Connecting to server... attempt 1
Connecting to server... attempt 2
Connecting to server... attempt 3
Gave up after 3 attempts.

What is the difference between while and do while?

The only difference is when the condition is checked. A while loop checks before the body; a do while loop checks after it. You can see it most clearly when the condition is false from the very start:

public class WhileVsDoWhile {
    public static void main(String[] args) {
        int tickets = 0;

        while (tickets > 0) {
            System.out.println("while: selling a ticket");
        }

        do {
            System.out.println("do while: checking for tickets once");
        } while (tickets > 0);
    }
}

do while: checking for tickets once

The while loop printed nothing. The do while loop ran its body once, then saw that tickets > 0 was false and stopped.

When does "at least once" matter?

Remember the digit counter from the while loop lesson? It has a hidden bug: for the number 0 it reports 0 digits, because 0 > 0 is false and the body never runs. A do while loop fixes it naturally, since every number has at least one digit:

public class DigitCountFixed {
    public static void main(String[] args) {
        int number = 0;
        int digits = 0;
        int remaining = number;

        do {
            remaining = remaining / 10;
            digits++;
        } while (remaining > 0);

        System.out.println(number + " has " + digits + " digit(s)");
    }
}

0 has 1 digit(s)

Building a menu with do while and Scanner

Menus are the classic do while job. The program must show the menu at least once, and it keeps showing it until the user picks "Exit". This mini banking app combines a do while loop, Scanner from Java user input and an arrow switch:

import java.util.Scanner;

public class BankMenu {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int balance = 100;
        int choice;

        do {
            System.out.println("1) Balance  2) Deposit $50  3) Withdraw $20  0) Exit");
            System.out.print("Choose an option: ");
            choice = input.nextInt();

            switch (choice) {
                case 1 -> System.out.println("Your balance is $" + balance);
                case 2 -> {
                    balance += 50;
                    System.out.println("Deposited $50.");
                }
                case 3 -> {
                    if (balance >= 20) {
                        balance -= 20;
                        System.out.println("Withdrew $20.");
                    } else {
                        System.out.println("Not enough money.");
                    }
                }
                case 0 -> System.out.println("Goodbye!");
                default -> System.out.println("Unknown option, try again.");
            }
        } while (choice != 0);
    }
}

Here's a sample run (the user typed 2, then 3, then 1, then 0):

1) Balance  2) Deposit $50  3) Withdraw $20  0) Exit
Choose an option: 2
Deposited $50.
1) Balance  2) Deposit $50  3) Withdraw $20  0) Exit
Choose an option: 3
Withdrew $20.
1) Balance  2) Deposit $50  3) Withdraw $20  0) Exit
Choose an option: 1
Your balance is $130
1) Balance  2) Deposit $50  3) Withdraw $20  0) Exit
Choose an option: 0
Goodbye!

Notice that choice is declared before the do. That's required, as you'll see in the common mistakes below.

How do you validate input with a do while loop?

Input validation means checking that the user typed something sensible and asking again if not. The do while shape fits perfectly: ask once, and repeat while the answer is invalid.

import java.util.Scanner;

public class DeliveryRating {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int stars;

        do {
            System.out.print("Rate your delivery (1-5): ");
            stars = input.nextInt();
            if (stars < 1 || stars > 5) {
                System.out.println("Please enter a number from 1 to 5.");
            }
        } while (stars < 1 || stars > 5);

        System.out.println("Thanks! You gave us " + stars + " stars.");
    }
}

Here's a sample run (the user typed 7, then 0, then 4):

Rate your delivery (1-5): 7
Please enter a number from 1 to 5.
Rate your delivery (1-5): 0
Please enter a number from 1 to 5.
Rate your delivery (1-5): 4
Thanks! You gave us 4 stars.

Without do while, you'd have to write the prompt and nextInt() twice: once before a while loop and again inside it. The do while version keeps it in one place.

Common mistakes with the Java do while loop

1. Forgetting the semicolon

Unlike a while loop, a do while loop must end with while (condition);. Leave out the semicolon and the compiler reports ';' expected.

2. Declaring the condition variable inside the body

do {
    int choice = input.nextInt();
} while (choice != 0);   // error: cannot find symbol

A variable declared inside the braces disappears at the closing }, before the condition is checked. Declare it before do, as in the menu example.

3. Using do while when zero runs is valid

If "do nothing" is a correct outcome, such as processing an empty shopping cart, a plain while loop is safer. Do while would process one item that doesn't exist.

4. A condition that never becomes false

Just like while, the body must change something the condition depends on. In a menu, that's reading a new choice each time.

Practice exercise

import java.util.Scanner;

public class AtmPin {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int correctPin = 2468;
        int pin;
        int attempts = 0;

        do {
            System.out.print("Enter PIN: ");
            pin = input.nextInt();
            attempts++;
        } while (pin != correctPin);

        System.out.println("Access granted after " + attempts + " attempt(s).");
    }
}

Enter PIN: 1234
Enter PIN: 2468
Access granted after 2 attempt(s).

What's next?

Sometimes you need to leave a loop early or skip one round. Next, learn Java break and continue to take finer control of any loop.