Loops: for, while, do-while and for-each

Loops let a program repeat work: process every order, retry a payment, print a report row for each student. Java has four loops, and choosing the right one makes code clearer. This lesson also covers the nested-loop patterns that appear in nearly every fresher coding test.

The for loop

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            System.out.println("7 x " + i + " = " + 7 * i);
        }
    }
}

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35

Any part can be different: count down with i--, step by 2 with i += 2, or use two variables for (int i = 0, j = n - 1; i < j; i++, j--) – the two-pointer technique used in many interview problems.

The while loop

Use while when you don't know in advance how many passes you need – you only know when to stop:

public class Main {
    public static void main(String[] args) {
        double savings = 10000;
        int months = 0;
        while (savings < 20000) {
            savings *= 1.01;    // 1% per month
            months++;
        }
        System.out.printf("Doubled in %d months (₹%.0f)%n", months, savings);
    }
}

Doubled in 70 months (₹20068)

The do-while loop

do-while checks the condition after the body, so the body always runs at least once. It fits menus and "ask until valid" input:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int choice;
        do {
            System.out.print("1) Balance 2) Deposit 3) Exit > ");
            choice = sc.nextInt();
            switch (choice) {
                case 1 -> System.out.println("Balance: ₹5000");
                case 2 -> System.out.println("Deposited");
                case 3 -> System.out.println("Goodbye");
                default -> System.out.println("Invalid option");
            }
        } while (choice != 3);
    }
}

1) Balance 2) Deposit 3) Exit > 1
Balance: ₹5000
1) Balance 2) Deposit 3) Exit > 9
Invalid option
1) Balance 2) Deposit 3) Exit > 3
Goodbye

The for-each loop

For walking through every element of an array or collection, the enhanced for is cleaner and avoids index mistakes:

import java.util.List;

public class Main {
    public static void main(String[] args) {
        int[] marks = {78, 92, 65, 88};
        int total = 0;
        for (int m : marks) {
            total += m;
        }
        System.out.println("Average: " + (double) total / marks.length);

        List<String> cities = List.of("Pune", "Delhi", "Chennai");
        for (String city : cities) {
            System.out.println(city.toUpperCase());
        }
    }
}

Average: 80.75
PUNE
DELHI
CHENNAI

Use a classic for instead when you need the index, want to go backwards, or need to modify array elements by position.

Which loop?

Nested loops and patterns

The inner loop completes fully for every pass of the outer loop. Pattern questions test whether you can turn a picture into rules: what does row r contain?

public class Main {
    public static void main(String[] args) {
        int n = 4;
        // Right triangle
        for (int r = 1; r <= n; r++) {
            for (int c = 1; c <= r; c++) System.out.print("* ");
            System.out.println();
        }
        // Pyramid: n - r spaces, then 2r - 1 stars
        for (int r = 1; r <= n; r++) {
            System.out.println(" ".repeat(n - r) + "*".repeat(2 * r - 1));
        }
        // Number triangle
        for (int r = 1; r <= n; r++) {
            for (int c = 1; c <= r; c++) System.out.print(c);
            System.out.println();
        }
    }
}

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* * *
* * * *
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Practice

Summary