Loops

Why Loops?

Imagine you have to write "I will not throw paper airplanes" 100 times on the board. You could write 100 System.out.println() lines... or you could use a loop and write it once.

Loops let your code repeat a block of instructions. Java gives you three flavors:

for — best when you know how many times to repeat.
while — best when you repeat until some condition changes.
do-while — like while, but guarantees at least one run.

The Classic for Loop

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

        System.out.println("---");

        // Countdown!
        for (int i = 3; i >= 1; i--) {
            System.out.println(i + "...");
        }
        System.out.println("Go!");

        System.out.println("---");

        // Skip by 2s (even numbers)
        for (int i = 2; i <= 10; i += 2) {
            System.out.print(i + " ");
        }
        System.out.println();
    }
}

Output

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
---
3...
2...
1...
Go!
---
2 4 6 8 10

While, Do-While & For-Each

A while loop is like a guard at a door who keeps checking: "Is the condition still true? OK, go again." It checks before each run, so if the condition is false from the start, the body never executes.

A do-while loop is the opposite — it runs the body first, then checks. Think of it like a restaurant that gives you one free sample before asking if you want more.

The enhanced for-each loop is the simplest way to go through every item in an array or collection. You don't need an index counter — Java hands you each item directly.

While, Do-While & For-Each

public class Main {
    public static void main(String[] args) {
        // While loop — keep halving until we're under 1
        double value = 100.0;
        int steps = 0;
        while (value >= 1) {
            value /= 2;
            steps++;
        }
        System.out.println("Took " + steps + " halvings to get below 1");
        System.out.println("Final value: " + value);

        System.out.println("---");

        // Do-while — always runs at least once
        int number = 0;
        do {
            System.out.println("This runs even though number is " + number);
            number++;
        } while (number < 0);  // condition is false, but body ran once!

        System.out.println("---");

        // Enhanced for-each — the cleanest loop
        String[] pets = {"Dog", "Cat", "Fish", "Hamster"};
        for (String pet : pets) {
            System.out.println("I have a " + pet);
        }
    }
}

Output

Took 7 halvings to get below 1
Final value: 0.78125
---
This runs even though number is 0
---
I have a Dog
I have a Cat
I have a Fish
I have a Hamster

Break, Continue & Labeled Break

public class Main {
    public static void main(String[] args) {
        // break — emergency exit from the loop
        System.out.print("Searching: ");
        int[] numbers = {3, 7, 2, 9, 4, 1};
        for (int n : numbers) {
            System.out.print(n + " ");
            if (n == 9) {
                System.out.println("\nFound 9! Stopping.");
                break;
            }
        }

        // continue — skip this round, go to the next one
        System.out.print("Odd numbers: ");
        for (int i = 1; i <= 10; i++) {
            if (i % 2 == 0) {
                continue;  // skip even numbers
            }
            System.out.print(i + " ");
        }
        System.out.println();

        // Labeled break — escape from nested loops
        System.out.println("Finding pair that sums to 7:");
        outer:
        for (int i = 1; i <= 5; i++) {
            for (int j = 1; j <= 5; j++) {
                if (i + j == 7) {
                    System.out.println(i + " + " + j + " = 7");
                    break outer;  // breaks BOTH loops
                }
            }
        }
    }
}

Output

Searching: 3 7 2 9 
Found 9! Stopping.
Odd numbers: 1 3 5 7 9 
Finding pair that sums to 7:
2 + 5 = 7

Why Loops?

The Classic for Loop

While, Do-While & For-Each

While, Do-While & For-Each

Break, Continue & Labeled Break

Quick check