Java While Loop
Syntax, Flow & Examples

In Java, a while loop is used to repeat a block of code as long as a given condition remains true. It offers flexibility and control, especially when the number of iterations isn't known beforehand.

Syntax of Java while Loop

while (condition) {
    // code block to execute
}

The loop checks the condition before every iteration. If the condition evaluates to true, the code block executes. Once the condition becomes false, the loop stops running.

Simple Example: Counting from 1 to 5

public class WhileLoopExample {
    public static void main(String[] args) {
        int i = 1;
        while (i <= 5) {
            System.out.println("Count: " + i);
            i++;
        }
    }
}

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

Explanation

Here, the loop starts with i = 1. The condition i <= 5 is checked before each iteration. As long as this is true, it prints the current value of i and increments it. When i becomes 6, the loop terminates.

Important: Infinite While Loop

If the condition never turns false, the loop keeps running indefinitely. This is called an infinite loop.

while (true) {
    System.out.println("This loop runs forever!");
}

Be cautious with infinite loops. They're useful in some scenarios like real-time systems or game loops, but can also crash your program if misused.

Using while Loop to Iterate over an Array

public class ArrayWhileLoop {
    public static void main(String[] args) {
        int[] nums = {10, 20, 30, 40, 50};
        int index = 0;
        
        while (index < nums.length) {
            System.out.println("Element at index " + index + ": " + nums[index]);
            index++;
        }
    }
}

Element at index 0: 10
Element at index 1: 20
Element at index 2: 30
Element at index 3: 40
Element at index 4: 50

Use Case: Validate User Input

The while loop is ideal when waiting for a correct user input:

import java.util.Scanner;

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

        while (!input.equals("yes")) {
            System.out.print("Type 'yes' to continue: ");
            input = scanner.nextLine();
        }

        System.out.println("Great! Moving forward...");
    }
}

Type 'yes' to continue: no
Type 'yes' to continue: maybe
Type 'yes' to continue: yes
Great! Moving forward...

When Should You Use a while Loop?

• When the number of iterations is unknown
• When looping depends on dynamic conditions (user input, system state)
• To wait or retry until a condition is met

Common Mistakes to Avoid

• Forgetting to update the condition inside the loop (may cause infinite loop)
• Using = instead of == for comparison
• Modifying the loop variable in an unexpected way