Java Loops

Q: What is the difference between while and do-while in Java?

while checks the condition before each run, so the body may never execute. do-while checks after, so the body always runs at least once.

Q: Can I modify a list inside a for-each loop in Java?

Adding or removing elements during a for-each over a collection throws ConcurrentModificationException. Use an Iterator with remove(), or removeIf().

Q: How do I stop an infinite loop in Java?

Make sure the loop condition eventually becomes false, or use break when a stop condition is met. In a terminal, press Ctrl+C to kill a running program.

Java loops repeat a block of code while a condition is true. They save you from writing the same statement again and again, and they are how you process every item in an array or list.

The for Loop

A for loop has three parts: start (int i = 1), condition (i <= 5) and update (i++). Use it when you know how many times to repeat.

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("3 x " + i + " = " + (3 * i));
        }
    }
}

The Enhanced for-each Loop

To visit every element of an array or collection, the for-each loop is shorter and avoids index mistakes.

public class Main {
    public static void main(String[] args) {
        String[] fruits = {"Mango", "Apple", "Banana"};
        for (String fruit : fruits) {
            System.out.println("I like " + fruit);
        }
    }
}

I like Mango
I like Apple
I like Banana

while and do-while Loops

A while loop checks its condition first and may run zero times. A do-while loop runs the body once before checking, so it always runs at least once.

public class Main {
    public static void main(String[] args) {
        int balance = 100;
        int years = 0;
        while (balance < 150) {
            balance += balance / 10; // 10% growth
            years++;
        }
        System.out.println(years + " years, balance " + balance);

        int n = 10;
        do {
            System.out.println("Runs once, n = " + n);
        } while (n < 5);
    }
}

5 years, balance 160
Runs once, n = 10

break and continue

break exits the loop immediately. continue skips the rest of the current iteration and moves to the next one.

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 10; i++) {
            if (i % 2 == 0) continue; // skip even numbers
            if (i > 7) break;         // stop after 7
            System.out.print(i + " ");
        }
        System.out.println();
    }
}

1 3 5 7

Nested Loops

A loop inside another loop runs completely for each step of the outer loop. This is useful for grids and tables.

public class Main {
    public static void main(String[] args) {
        for (int row = 1; row <= 3; row++) {
            for (int col = 1; col <= row; col++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

*
**
***

Frequently asked questions

What is the difference between while and do-while in Java?

while checks the condition before each run, so the body may never execute. do-while checks after, so the body always runs at least once.

Can I modify a list inside a for-each loop in Java?

Adding or removing elements during a for-each over a collection throws ConcurrentModificationException. Use an Iterator with remove(), or removeIf().

How do I stop an infinite loop in Java?

Make sure the loop condition eventually becomes false, or use break when a stop condition is met. In a terminal, press Ctrl+C to kill a running program.