while Loop

Repeat code while a condition is true - ideal when the number of iterations is not known in advance.

Overview

The while loop repeats a block of code as long as a condition remains true. Unlike the for loop (which is best when the iteration count is known upfront), the while loop excels when you repeat until something changes - reading input until the user types "quit", retrying until a network succeeds, or processing data until you reach the end. The condition is checked before each iteration. If it is false from the start, the body never runs. You must ensure the condition eventually becomes false, or the loop will run forever.

Key Points

•Syntax: while (condition) { ... } - condition checked before each iteration
•If condition is false at the start, the body never executes (zero iterations)
•The body must eventually change the condition to false, or you get an infinite loop
•Common pattern: while (scanner.hasNextInt()) - process until input ends
•while is preferred over for when the number of iterations is unknown beforehand
•Use a flag variable (boolean running = true;) to control complex loop exit conditions

Examples

Count up with while

public class Main {
    public static void main(String[] args) {
        int count = 1;

        while (count <= 5) {
            System.out.println("Count: " + count);
            count++; // update - must be inside the body!
        }
        System.out.println("Done");
    }
}

The update (count++) is inside the body. Forgetting it creates an infinite loop.

Input validation loop

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int number = -1;

        while (number < 1 || number > 10) {
            System.out.print("Enter a number between 1 and 10: ");
            number = sc.nextInt();
            if (number < 1 || number > 10) {
                System.out.println("Invalid! Try again.");
            }
        }

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

This loop keeps asking until valid input is received - classic "input validation" pattern.

Finding the first power of 2 greater than 1000

public class Main {
    public static void main(String[] args) {
        int power = 1;

        while (power <= 1000) {
            power *= 2;
        }

        System.out.println("First power of 2 > 1000: " + power); // 1024
    }
}

The loop count is unknown in advance - this is exactly when while outshines for.

Common Mistakes

Infinite loop: condition never becomes false because the variable inside it is never updated
Unreachable code after an infinite while(true) loop - the compiler may warn about this
Checking condition before any value is set: initialise the variable to a value that guarantees at least one iteration when needed

Note

A while(true) loop with a break statement inside is a common pattern for menus and game loops: while (true) { ... if (quit) break; }. This makes the exit condition explicit and visible.

while Loop

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

When is the while loop condition checked?