While & Do-While Loops

Beginner ~15 min read

While for loops are great when you know the number of iterations, while loops are perfect when you want to loop until a condition changes.

The `while` Loop

It checks the condition before entering the loop block. If the condition is false initially, the code never runs.

while (condition) {
    // Code to execute
    // Update variables
}

The `do-while` Loop

It checks the condition after executing the block. This guarantees the code runs at least once.

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

Use Case: Use do-while for user menus (display menu, get choice, repeat if invalid). You always want to show the menu at least once.

public class WhileLoops {
    public static void main(String[] args) {
        // While Loop Demo
        System.out.println("--- While Loop (Guessing Game Logic) ---");
        int target = 5;
        int current = 0;

// Simulating finding a number (0 to 10)
        while (current != target) {
            System.out.println("Guessed: " + current);
            current++;
        }
        System.out.println("Found target: " + current);

// Do-While Loop Demo
        System.out.println("\n--- Do-While Loop (Menu Logic) ---");
        int count = 1;
        do {
            System.out.println("Processing item " + count);
            count++;
        } while (count <= 3); // Condition checked at end

// While vs Do-While difference
        System.out.println("\n--- Difference Demo ---");
        boolean condition = false;

while (condition) {
            System.out.println("This (While) will NOT print.");
        }

do {
            System.out.println("This (Do-While) WILL print once.");
        } while (condition);
    }
}

Output

Click Run to execute your code

The Infinite Loop Trap

If you forget to update the condition variable inside the loop, it might never end!

int i = 0;
while (i < 5) {
    System.out.println(i);
    // Missing i++ here! Loop runs forever.
}

Summary

While Loop: Checks condition first. Might run 0 times.
Do-While Loop: Checks condition last. Runs at least 1 time.
Always ensure your loop has a way to exit (avoid infinite loops).

Previous For Loops

Next Loop Control →

The while Loop

The do-while Loop

The Infinite Loop Trap

Summary

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The `while` Loop

The `do-while` Loop