INTRODUCTION

Remember when we talked about nested if-else statements ? One structure of if-else inside another structure. Similarly here we will have loop inside another loop. It'll come handy in many projects and actual coding later. A student who understands nested loops are often considered good in programming. 


NESTED LOOPS

Suppose a monkey wants 5 bananas. For each banana it has to perform in circus. So it will be a loop of performing 5 times in circus. Now imagine the performance it gives in circus is jumping into a fire ring 5 times. This jumping into fire ring is also a loop of single act that it will do 5 times.
So here we have a loop of working in circus. In each iteration it'll run another loop of jumping into the ring 5 times. In a single iteration of one loop, another loop runs and completes as many times as we set. This is the concept of nested loops.

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!"); 
    }
} 

The output will be: 
 Monkey performed in circus  
 Banana acquired: 1  
 Monkey performed in circus  
 Banana acquired: 2  
 Monkey performed in circus  
 Banana acquired: 3  
 Monkey performed in circus  
 Banana acquired: 4  
 Monkey performed in circus  
 Banana acquired: 5  
 Loop exited!  

This is simple one loop, now let's write a loop for the performance, i.e. in each performance monkey have 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!");

                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 output will be: 
 Performance starts!           (Outer loop at 1st iteration)
 Monkey jumped 1 time(s)  (Inner loop at 1st iteration)
 Monkey jumped 2 time(s)  (Inner loop at 2nd iteration)
 Monkey jumped 3 time(s)  (Inner loop at 3srd iteration)
 Monkey jumped 4 time(s)  (Inner loop at 4th iteration)
 Monkey jumped 5 time(s)  (Inner loop at last iteration)
 Performance ended! (Inner Loop exited)  
 Banana acquired: 1  

 Performance starts!           (Outer loop at 2nd iteration)
 Monkey jumped 1 time(s)  (Inner loop at 1st iteration)
 Monkey jumped 2 time(s)  (Inner loop at 2nd iteration)
 Monkey jumped 3 time(s)  (Inner loop at 3rd iteration)
 Monkey jumped 4 time(s)  (Inner loop at 4th iteration)
 Monkey jumped 5 time(s)  (Inner loop at last iteration)
 Performance ended! (Inner Loop exited)  
 Banana acquired: 2  

 Performance starts!           (Outer loop at 3rd iteration)
 Monkey jumped 1 time(s)  (Inner loop at 1st iteration)
 Monkey jumped 2 time(s)  (Inner loop at 2nd iteration)
 Monkey jumped 3 time(s)  (Inner loop at 3rd iteration)
 Monkey jumped 4 time(s)  (Inner loop at 4th iteration)
 Monkey jumped 5 time(s)  (Inner loop at last iteration)
 Performance ended! (Inner Loop exited)  
 Banana acquired: 3  

 Performance starts!           (Outer loop at 4th iteration)
 Monkey jumped 1 time(s)  (Inner loop at 1st iteration)
 Monkey jumped 2 time(s)  (Inner loop at 2nd iteration)
 Monkey jumped 3 time(s)  (Inner loop at 3rd iteration)
 Monkey jumped 4 time(s)  (Inner loop at 4th iteration)
 Monkey jumped 5 time(s)  (Inner loop at last iteration)
 Performance ended! (Inner Loop exited)  
 Banana acquired: 4  

 Performance starts!           (Outer loop at last iteration)
 Monkey jumped 1 time(s)  (Inner loop at 1st iteration)
 Monkey jumped 2 time(s)  (Inner loop at 2nd iteration)
 Monkey jumped 3 time(s)  (Inner loop at 3rd iteration)
 Monkey jumped 4 time(s)  (Inner loop at 4th iteration)
 Monkey jumped 5 time(s)  (Inner loop at last iteration)
 Performance ended! (Inner Loop exited)  
 Banana acquired: 5  
 All performances completed! (Outer Loop exited)  

Explanation
⦿ The outer loop runs 5 times → Once for each banana.
⦿ For each banana inner loop runs 5 times → 5 jumps
⦿ Total jumps = 5 x 5 = 25
⦿ In each iteration of outer loop, inner loop freshly starts from 1 and end 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);

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

        // Generate and print matrix
        int value = 1; // start filling from 1
        for (int i = 1; i <= n; i++) { // rows
            for (int j = 1; j <= n; j++) { // columns
                System.out.print(value + "\t"); 
                value++;
            }
            System.out.println(); // move to next row
        }
    }
}

For input  3  the output will be: 
 1    2    3  
 4    5    6  
 7    8    9  

⦿ The outer loop controls number of rows. On each iteration it allows inner loop to run and do whatever it wants, at end of iteration it shifts to next line. And that's how it control rows.

⦿ The inner loop actually prints numbers on the row set by outer loop, it runs all of its iterations and print the numbers on single line. 

⦿ Each value inner loop print act as separate colum on that row. Therefore we say inner loop controls number of columns.

⦿ Each time a value is printed  value++;  updates the value so that next value is NOT same as previous

⦿ The  n  in condition controls how many iteration outer loop and inner loop will run. If user enter 3, both inner and outer loop will run for 3 iterations. As result outer loop will create 3 rows, and inner loop will create 3 columns on it



CLOSING
That's it for  Nested Loops  in java. You will be now able to understand youtube tutorials and text books. There are few complex things in nested loops though, e.g. pattern making like this:

Right Triangle:
* 
* * 
* * * 
* * * * 
* * * * * 

Pyramid:
        * 
      * * * 
    * * * * * 
  * * * * * * * 
* * * * * * * * * 

Hollow Triangle:
* 
* * 
*   * 
*     * 
* * * * * 

Hollow Square:
* * * * * 
*         * 
*         * 
*         * 
* * * * * 

But now you'll be able to understand them from tutorials. Still if there is any issue, you can ask me or any other senior on campus. I assure you they are all really nice.