Do While Loop

A school program needs reliable data and clear rules. A do-while loop runs its body before checking whether to repeat. This is useful for an action that must happen at least once.

In Java, unlike the while loop, which checks the condition before executing the code block, the do-while loop guarantees at least one execution—no matter what the condition says. It's a perfect fit when your logic requires the body to run before validation kicks in.

A do-while loop in Java is a control flow statement that executes a block of code at least once, and then continues to execute it as long as the specified condition is true.

do {
    // Code to execute
} while (condition);

Note: The semicolon after the while condition is required. This often trips up beginners, so keep an eye on it!

public class Main {
    public static void main(String[] args) {
        int count = 1;
        do {
            System.out.println("Count is: " + count);
            count++;
        } while (count <= 3);
    }
}

Count is: 1
Count is: 2
Count is: 3

Here’s what happens line-by-line:

• count starts at 1.
• The do block runs and prints “Count is: 1”.
• count increments to 2, and the loop condition (count <= 3) is checked.
• Since it's still true, the block runs again. This continues until count becomes 4.
• At count = 4, the condition fails, and the loop exits.

The do-while loop shines in scenarios where the loop body must execute at least once before the condition is checked. For example:

import java.util.Scanner;

public class LoginPrompt {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String password;
        
        do {
            System.out.print("Enter your password: ");
            password = scanner.nextLine();
        } while (!password.equals("admin123"));
        
        System.out.println("Access granted.");
    }
}

Enter your password: user1
Enter your password: hello
Enter your password: admin123
Access granted.

This loop ensures that the user is always prompted at least once. The moment the correct password is entered, it breaks out of the loop.

public class Main {
    public static void main(String[] args) {
        int x = 100;
        do {
            System.out.println("This will run once even if condition is false.");
        } while (x < 50);
    }
}

This will run once even if condition is false.

Unlike while or for loops, a do-while will not skip execution—even if the condition is false from the beginning.

• Use do-while when the logic demands at least one execution.
• Make sure the loop condition eventually becomes false to avoid infinite loops.
• Use meaningful loop conditions—especially in user input scenarios.

The Java do-while loop is more than just a variation of while; it’s a purposeful construct tailored for specific use cases. When you're certain that your block of code needs to run at least once, do-while loop is the one you have to use.

Even a false initial continuation condition cannot prevent the first body execution. The trailing semicolon belongs to this syntax.

Before running this Java program, write down each line you expect to see. Identify the value, branch, object or exception responsible for each line. Then explain whether the following assumption is valid: Even a false initial continuation condition cannot prevent the first body execution.

Java 17 · Main.java · complete practice program

public class Main  {
  public static void main(String[] args)  {
    int attempts = 0;
    do  {
      attempts++;
    }
    while (attempts < 0);
    System.out.println(attempts);
  }
}

Now change one thing: Start attempts at 5. Predict the new behavior before checking the worked variation.

Java 17 · Main.java · source repeated for this answer

public class Main  {
  public static void main(String[] args)  {
    int attempts = 0;
    do  {
      attempts++;
    }
    while (attempts < 0);
    System.out.println(attempts);
  }
}

Expected output

1

A do-while loop runs its body before checking whether to repeat. This is useful for an action that must happen at least once.

Even a false initial continuation condition cannot prevent the first body execution. The trailing semicolon belongs to this syntax.

Run with javac Main.java, then java Main. The declaration of Main names the class; main starts the example, and println writes a complete output line.

Worked variation: Start attempts at 5. The body still runs once and prints 6.

A do-while loop runs its body before checking whether to repeat. This is useful for an action that must happen at least once. Even a false initial continuation condition cannot prevent the first body execution. The trailing semicolon belongs to this syntax.

Related explanations: Java Variables, Java If Statement - Syntax and Examples, Java Class - Blueprint of Objects, Exceptions in Java Introduction.

Supporting documentation: Official Java learning guide and Java 17 language specification.