Loops

Java provides several kinds of loops: for, for-each (enhanced for), while, and do-while. Each kind has a specific use depending on the iteration needs.

The for loop is a good fit when the number of iterations is known upfront. The for-each loop is the most compact way to iterate over elements of an array or collection — no index needed. The while loop repeats as long as the condition is true, making it a good fit for iterations that depend on a runtime condition. do-while guarantees at least one execution before checking the condition, which is a good fit for input validation.

Java 5+ supports the enhanced for-each with the syntax for (Type element : array). The break operator stops the loop, and continue skips the current iteration and moves to the next one. Labels can be used to control nested loops.

Types of Loops:

• for — the number of iterations is known
• for-each — iterate over array/collection elements
• while — repeat while the condition is true
• do-while — at least one execution first

Code Example

public class Loops {
    public static void main(String[] args) {
        // For loop
        System.out.println("=== For Loop ===");
        for (int i = 1; i <= 5; i++) {
            System.out.println("Iteration " + i);
        }

        // For-each loop
        System.out.println("\n=== For-Each Loop ===");
        String[] fruits = {"apple", "mango", "orange"};
        for (String item : fruits) {
            System.out.println("Fruit: " + item);
        }

        // While loop
        System.out.println("\n=== While Loop ===");
        int count = 1;
        while (count <= 5) {
            System.out.println("Count: " + count);
            count++;
        }

        // Do-while loop
        System.out.println("\n=== Do-While Loop ===");
        int number = 10;
        do {
            System.out.println("Number: " + number);
            number++;
        } while (number < 5); // still executes once even though the condition is false
    }
}

Code Explanation

1. For loop: for (int i = 1; i <= 5; i++) — initialize i=1, check the condition i<=5, and increment i++ after each iteration.
2. For-each: for (String item : fruits) — automatically takes each element of the fruits array and stores it in the item variable.
3. While loop: the loop continues as long as count <= 5. Note that count++ must be inside the loop to avoid an infinite loop.
4. Do-while: the code block runs first, and only then is the condition checked. Even though number=10 is already greater than 5, the first line still prints "Number: 10".

Everyday Example

A program that prints a multiplication table using a nested for loop:

public class MultiplicationTable {
    public static void main(String[] args) {
        int n = 5;
        for (int i = 1; i <= n; i++) {
            StringBuilder line = new StringBuilder();
            for (int j = 1; j <= n; j++) {
                line.append(String.format("%4d", i * j));
            }
            System.out.println("Multiplication " + i + ":" + line);
        }
    }
}

Summary

Loops enable repeated code execution efficiently. Pick the right kind of loop: for for counter-based iteration, for-each for arrays/collections, while for dynamic conditions, and do-while for input validation. Avoid infinite loops by making sure the terminating condition is eventually reached.