Lesson 25. The do-while Loop in Java | Learn Basic Java

What is the do-while loop?

In Java programming, the do-while loop is one of the three fundamental loop types, alongside for and while. It is used when you want the block of code to execute at least once before checking the loop condition.

The do-while loop has a simple syntax and is useful in many situations, such as prompting users for input until valid data is entered or performing an action at least once before evaluating whether to continue.

Syntax of the do-while loop in Java

The syntax of the do-while loop in Java is as follows:

do {
    // The block of code is executed at least once
} while (condition);

How the do-while loop works:

1. The code inside do executes at least once.
2. After execution, the condition inside while is checked.
3. If the condition is true, the loop continues.
4. If the condition is false, the loop terminates.

Basic Examples of the do-while Loop

Below are two examples of the do-while loop:

1. A do-while loop where the condition is false from the beginning
2. A do-while loop for validating user input

Example 1: do-while Loop with an Initially False Condition

Even though the condition is false (i < 5 is false at the start), the loop still executes at least once.

public class DoWhileWrongCondition {
    public static void main(String[] args) {
        int i = 10; // i is initialized with a value that does not meet the condition

        do {
            System.out.println("The do-while loop runs once!");
        } while (i < 5); // Condition is false from the start

        System.out.println("The do-while loop has ended.");
    }
}

Output:

The do-while loop runs once!
The do-while loop has ended.

Explanation:

• Even though i = 10 does not satisfy i < 5, the loop still runs once before checking the condition.
• After the first execution, the condition is false, so the loop terminates.

Example 2: do-while Loop for User Input Validation

The program prompts the user to enter a number between 1 and 10. If the input is incorrect, the user is asked to enter it again.

import java.util.Scanner;

public class DoWhileUserInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int number;

        do {
            System.out.print("Enter a number between 1 and 10: ");
            number = scanner.nextInt();

            if (number < 1 || number > 10) {
                System.out.println("Invalid number! Please try again.");
            }
        } while (number < 1 || number > 10); // Condition check

        System.out.println("You entered a valid number: " + number);
        scanner.close();
    }
}

Example program execution:

Enter a number between 1 and 10: 15
Invalid number! Please try again.
Enter a number between 1 and 10: 0
Invalid number! Please try again.
Enter a number between 1 and 10: 7
You entered a valid number: 7

Explanation:

• The program always runs at least once to prompt for input.
• If the number is outside the range of 1-10, the program asks for input again.
• When a valid number is entered, the loop stops and displays the result.

Comparison of do-while and while loops

Key Differences:

Real-World Applications of do-while Loop

Prompting User Input Until Valid Data is Entered

Suppose you want to ask the user for a number between 1 and 10, and if they enter an invalid number, they must try again.

import java.util.Scanner;

public class InputValidation {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int number;

        do {
            System.out.print("Enter a number between 1 and 10: ");
            number = scanner.nextInt();
        } while (number < 1 || number > 10); // Condition check

        System.out.println("You entered a valid number: " + number);
        scanner.close();
    }
}

How it works:

• The loop runs at least once, prompting for input.
• If the entered number is out of range (1-10), the program asks for input again.
• When a valid number is entered, the loop stops and prints the valid input.

Calculating Sum Until the User Enters 0

import java.util.Scanner;

public class SumUntilZero {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int number;
        int sum = 0;

        do {
            System.out.print("Enter a number (0 to stop): ");
            number = scanner.nextInt();
            sum += number; // Add input to total sum
        } while (number != 0); // Stop when 0 is entered

        System.out.println("Total sum of entered numbers: " + sum);
        scanner.close();
    }
}

How it works:

• The program asks for numbers multiple times.
• If the user enters 0, the loop stops and prints the total sum.

When Should You Use the do-while Loop?

Use do-while in situations like:

• When you need to ensure the block executes at least once.
• When prompting user input validation and requiring a valid entry.
• When handling menu-driven programs in console applications.

Avoid using do-while when:

• You aren't sure if the loop should run at least once.
• You need to check the condition before executing the loop.

Conclusion

In summary: The do-while loop is useful when you need to execute the block at least once before checking the condition.

Next Article: Lesson 26: Using break, continue, and return in Java