Lesson 23. Guide to Using the While Loop in Java | Learn Java Basics

The while loop is one of the three main loop types in Java. It allows you to repeatedly execute a block of code as long as a condition remains true. This is an essential tool in programming, helping automate repetitive tasks without needing to write the same code multiple times.

What is the While Loop?

The while loop is a conditional loop in Java, meaning it continues looping as long as the condition remains true (true). Once the condition becomes false, the loop stops.

When to Use the While Loop?

• When you don’t know the exact number of iterations but only know the stopping condition.
• When you need to check the condition before executing the loop body.
• When you need an infinite loop with a stopping condition inside the loop.

Syntax of the While Loop in Java

while (condition) {
    // Code block to be executed while the condition is true
}

Explanation:

• condition: A boolean expression that evaluates to true or false.
• If condition is true, the code inside {} executes.
• If condition is false, the loop terminates immediately.

How the While Loop Works

1. The condition (condition) is evaluated.
2. If the condition is true, the code inside {} executes.
3. The condition is checked again.
4. If the condition becomes false, the loop terminates.

Note: If the condition never becomes false, the loop runs indefinitely (infinite loop).

Examples of the While Loop in Java

Basic While Loop

Example: Printing numbers from 1 to 5.

public class WhileExample {
    public static void main(String[] args) {
        int i = 1; // Initialize counter

        while (i <= 5) { // Check condition
            System.out.println("Number: " + i);
            i++; // Increment counter
        }
    }
}

Output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

While Loop for Valid User Input

Example: Prompting the user to enter a number greater than 0.

import java.util.Scanner;

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

        System.out.print("Enter a number greater than 0: ");
        number = scanner.nextInt();

        while (number <= 0) {
            System.out.print("Invalid number! Try again: ");
            number = scanner.nextInt();
        }

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

Output (if the user enters an incorrect value first):

Enter a number greater than 0: -3
Invalid number! Try again: 5
You entered: 5

Infinite While Loop (Exit on 'exit' Input)

Example: Continuously prompting the user for input until they type "exit":

import java.util.Scanner;

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

        while (true) { // Infinite loop
            System.out.print("Enter a word (or 'exit' to quit): ");
            input = scanner.nextLine();

            if (input.equalsIgnoreCase("exit")) {
                System.out.println("Exiting program.");
                break; // Exit the loop
            }

            System.out.println("You entered: " + input);
        }

        scanner.close();
    }
}

Output:

Enter a word (or 'exit' to quit): hello
You entered: hello
Enter a word (or 'exit' to quit): java
You entered: java
Enter a word (or 'exit' to quit): exit
Exiting program.

Advantages and Disadvantages of the While Loop

Advantages:

• Simple and easy to use when the number of iterations is unknown.
• Can run indefinitely if no stopping condition is provided.
• Efficient for handling user input validation.

Disadvantages:

• Can cause an infinite loop if the condition never changes.
• Less control over iterations compared to the for loop.

Comparison Between While and Do-While Loops

Conclusion

The while loop is a powerful tool in Java, allowing you to perform repetitive tasks when the number of iterations is unknown. Always ensure the loop condition updates properly to avoid infinite loops.

Next Lesson: Lesson 24. How to Convert Decimal to Binary in Java