11 Java Fundamentals

Nested Loops in Java

By Dummy for Dummies

Introduction

Remember nested if-else statements? One structure sitting inside another. Same idea here, except now it's a loop inside a loop. You'll run into this constantly once you start building real programs, so it's worth getting comfortable with early.

Nested Loops

Suppose a monkey wants 5 bananas. For each banana, it has to perform in the circus, so that's a loop running 5 times. Now say the performance itself is jumping through a fire ring, and it does that 5 times per performance too. That jump is its own loop, nested inside the performance loop.

So the outer loop handles the circus performances. Each time it runs once, the inner loop runs its full 5 jumps before the outer loop moves to the next performance. That's nested loops: one loop completing in full, inside a single step of another.

Here is a demonstration of just one loop, monkey performing and getting banana:

public class Main {
    public static void main(String[] args) {
        // Monkey performing and getting banana
        for (int banana = 1; banana < 6; banana++) {
            System.out.println("Monkey performed in circus");
            System.out.println("Banana acquired: " + banana);
        }
        System.out.println("Loop exited!");
    }
}

This is a simple one loop.

Now let's write a loop for the performance, i.e., in each performance the monkey has to jump into the ring 5 times, so we will replace the performance line of code with a loop:

public class Main {
    public static void main(String[] args) {
        // Monkey performing and getting banana
        for (int banana = 1; banana < 6; banana++) {
            System.out.println("Performance starts!");

            // Inner loop: monkey jumps 1 to 5 times
            for (int jump = 1; jump < 6; jump++) {
                System.out.println("Monkey jumped " + jump + " time(s)");
            }

            System.out.println("Performance ended! (Inner Loop exited!)");
            System.out.println("Banana acquired: " + banana);
            System.out.println();
        }
        System.out.println("All performances completed! (Outer Loop exited)");
    }
}

• The outer loop runs 5 times → once for each banana
• For each banana, the inner loop runs 5 times → 5 jumps
• Total jumps = 5 x 5 = 25
• In each iteration of the outer loop, the inner loop freshly starts from 1 and ends at 5.

Technical Example (Matrix)

Let's write a small code that outputs a square matrix of any order we want.

import java.util.Scanner;

public class SquareMatrix {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter order of square matrix (e.g., 3 for 3x3): ");
        int n = scanner.nextInt();

        int value = 1;
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n; j++) {
                System.out.print(value + "\t");
                value++;
            }
            System.out.println();
        }

        scanner.close();
    }
}

• The outer loop controls the number of rows. On each iteration, it allows the inner loop to run and do whatever it wants. At the end of the iteration, it shifts to the next line. That's how it controls rows.
• The inner loop actually prints numbers on the row set by the outer loop. It runs all of its iterations and prints the numbers on a single line.
• Each value the inner loop prints acts as a separate column on that row. Therefore, we say the inner loop controls the number of columns.
• Each time a value is printed, value++ updates the value so that the next value is NOT the same as the previous.
• The n in the condition controls how many iterations the outer loop and inner loop will run. If the user enters 3, both inner and outer loops will run for 3 iterations. As a result, the outer loop will create 3 rows, and the inner loop will create 3 columns on each.

Closing

That's it for Nested Loops in Java. You will now be able to understand YouTube tutorials and textbooks. There are a few complex things in nested loops though, e.g., pattern making like this:

But now you'll be able to make sense of them from tutorials. For practice, try building the patterns above yourself. I'd suggest the Apna College YouTube channel for this. And if you get stuck, ask me or any senior on campus, they're all genuinely helpful.