Java Loops

The for loop is the most common loop in Java. Use it when you know how many times you want to repeat. It packs three things into one line: where to start, when to stop, and how to count.

// Anatomy of a for loop:
// for (start; keepGoing?; afterEach) { ... }

for (int i = 0; i < 5; i++) {
    System.out.println("i = " + i);
}
// Output: i = 0, i = 1, i = 2, i = 3, i = 4

// Let's break it down:
// int i = 0    → Start: create counter at 0
// i < 5        → Condition: keep going while i is less than 5
// i++          → Update: add 1 to i after each loop

Step-by-step execution:

1. int i = 0 — create the counter (runs once)
2. i < 5 → true → run the body → print "i = 0"
3. i++ → i becomes 1
4. i < 5 → true → run the body → print "i = 1"
5. ...continues until i reaches 5
6. 5 < 5 → false → loop ends

// Count by 2s (even numbers)
for (int i = 0; i <= 10; i += 2) {
    System.out.print(i + " ");  // 0 2 4 6 8 10
}

// Count backwards
for (int i = 10; i >= 0; i--) {
    System.out.print(i + " ");  // 10 9 8 7 6 5 4 3 2 1 0
}

The while loop checks its condition before each iteration. If the condition is false from the start, the body never runs at all. Use it when you don't know in advance how many iterations you need.

// Find the first power of 2 greater than 1000
int power = 1;
while (power <= 1000) {
    power *= 2;  // Double it each time
}
System.out.println(power);  // 1024

// How many times did it loop? We didn't know in advance!
// It ran 10 times (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024)

Common use case: Reading user input until they type "quit":

Scanner scanner = new Scanner(System.in);
String input = "";
while (!input.equals("quit")) {
    System.out.print("Enter command (or 'quit'): ");
    input = scanner.nextLine();
    System.out.println("You typed: " + input);
}

The do-while loop is like a while loop, but it checks the condition after running the body. This means the body always runs at least once, even if the condition is false. It's perfect for menu systems and input validation.

// Menu system — always show at least once
int choice;
do {
    System.out.println("=== Game Menu ===");
    System.out.println("1. Play");
    System.out.println("2. Settings");
    System.out.println("3. Quit");
    System.out.print("Choose: ");
    choice = scanner.nextInt();
} while (choice != 3);

// Input validation — keep asking until valid
int number;
do {
    System.out.print("Enter a positive number: ");
    number = scanner.nextInt();
} while (number <= 0);
System.out.println("You entered: " + number);

When you want to go through every item in an array or collection, the for-each loop is the cleanest option. No counter variable, no index, no chance of an off-by-one error.

String[] fruits = {"Apple", "Banana", "Cherry", "Date"};

// For-each: clean and simple
for (String fruit : fruits) {
    System.out.println("I like " + fruit);
}
// Reads as: "for each fruit in fruits"

// Compare with traditional for:
for (int i = 0; i < fruits.length; i++) {
    System.out.println("I like " + fruits[i]);
}
// Same result, but more code and more room for bugs

Sometimes you need to exit a loop early or skip certain iterations. Java gives you two tools:

// Search for a value
int[] data = {10, 20, 30, 40, 50};
for (int i = 0; i < data.length; i++) {
    if (data[i] == 30) {
        System.out.println("Found at " + i);
        break;  // Stop searching
    }
}
// Only checks 3 items, not all 5

// Sum only positive numbers
int[] nums = {5, -3, 8, -1, 12};
int sum = 0;
for (int n : nums) {
    if (n < 0) continue; // Skip negatives
    sum += n;
}
System.out.println("Sum: " + sum);
// Sum: 25 (5 + 8 + 12)

You can put a loop inside another loop. The inner loop runs completely for each iteration of the outer loop. If the outer loop runs 3 times and the inner loop runs 4 times, the body runs 3 × 4 = 12 times total.

// Multiplication table (3×3)
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.print(i * j + "\t");
    }
    System.out.println();  // New line after each row
}
// Output:
// 1   2   3
// 2   4   6
// 3   6   9

Common uses: 2D grids (game boards, images), generating tables, comparing every pair of items, building patterns.

// Star pyramid pattern
for (int row = 1; row <= 5; row++) {
    for (int star = 0; star < row; star++) {
        System.out.print("* ");
    }
    System.out.println();
}
// *
// * *
// * * *
// * * * *
// * * * * *

An infinite loop is a loop that never stops. Your program freezes, your CPU spins at 100%, and nothing else can happen. It's the most common loop bug:

// ❌ BUG: i never changes, so i < 10 is always true!
int i = 0;
while (i < 10) {
    System.out.println("Stuck forever...");
    // Forgot i++; !
}

// ❌ BUG: Wrong direction — i goes up but condition checks down
for (int i = 10; i >= 0; i++) {  // Should be i--
    System.out.println(i);
}

// ✅ Intentional infinite loop (used in servers and game loops)
while (true) {
    String command = getInput();
    if (command.equals("exit")) break;  // Manual exit
    processCommand(command);
}

Here's a simple decision guide: