If a loop exists inside the body of another loop, it's called a nested loop. Here's an example of the nested for loop.

// outer loop
for (int i = 1; i <= 5; ++i) {
  // codes

  // inner loop
  for(int j = 1; j <=2; ++j) {
    // codes
  }
..
}

Here, we are using a for loop inside another for loop.

We can use the nested loop to iterate through each day of a week for 3 weeks.

In this case, we can create a loop to iterate three times (3 weeks). And, inside the loop, we can create another loop to iterate 7 times (7 days).

Example 1: Java Nested for Loop

class Main {
  public static void main(String[] args) {

    int weeks = 3;
    int days = 7;

    // outer loop prints weeks
    for (int i = 1; i <= weeks; ++i) {
      System.out.println("Week: " + i);

      // inner loop prints days
      for (int j = 1; j <= days; ++j) {
        System.out.println("  Day: " + j);
      }
    }
  }
}

Output

Week: 1
  Day: 1
  Day: 2
  Day: 3
    .....  ..  ....
Week: 2
  Day: 1
  Day: 2
  Day: 3
  ....  ..  ....
....  .. ....

In the above example, the outer loop iterates 3 times and prints 3 weeks. And, the inner loop iterates 7 times and prints the 7 days.

We can also create nested loops with while and do...while in a similar way.

Note: It is possible to use one type of loop inside the body of another loop. For example, we can put a for loop inside the while loop.

Example 2: Java for loop inside the while loop

class Main {
  public static void main(String[] args) {

    int weeks = 3;
    int days = 7;
    int i = 1;

    // outer loop
    while (i <= weeks) {
      System.out.println("Week: " + i);

      // inner loop
      for (int j = 1; j <= days; ++j) {
        System.out.println("  Days: " + j);
      }
      ++i;
    }
  }
}

Output:

Week: 1
  Day: 1
  Day: 2
  Day: 3
  .... .. ....
Week: 2
  Day: 1
  Day: 2
  Day: 3
  .... .. ....
.... .. ....

Here you can see that the output of both Example 1 and Example 2 is the same.

Example 3: Java nested loops to create a pattern

We can use the nested loop in Java to create patterns like full pyramid, half pyramid, inverted pyramid, and so on.

Here is a program to create a half pyramid pattern using nested loops.

class Main {
  public static void main(String[] args) {

    int rows = 5;

    // outer loop
    for (int i = 1; i <= rows; ++i) {

      // inner loop to print the numbers
      for (int j = 1; j <= i; ++j) {
        System.out.print(j + " ");
      }
      System.out.println("");
    }
  }
}

Output

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

To learn more, visit the Java program to print pyramid and patterns.

break and continue Inside Nested Loops

When we use a break statement inside the inner loop, it terminates the inner loop but not the outer loop. For example,

class Main {
  public static void main(String[] args) {
    
    int weeks = 3;
    int days = 7;

    // outer loop
    for(int i = 1; i <= weeks; ++i) {
      System.out.println("Week: " + i);

      // inner loop
      for(int j = 1; j <= days; ++j) {
        
        // break inside the inner loop
        if(i == 2) {
          break;
        }
        System.out.println("  Days: " + j);
      }
    }
  }
}

Output

Week: 1
  Day: 1
  Day: 2
  .... .. ....
Week: 2
Week: 3
  Day: 1
  Day: 2
  .... .. ....
.... .. ....

In the above example, we have used the break statement inside the inner for loop. Here, the program skips the loop when i is 2.

Hence, days for week 2 are not printed. However, the outer loop that prints week is unaffected.

Similarly, when we use a continue statement inside the inner loop, it skips the current iteration of the inner loop only. The outer loop is unaffected. For example,

class Main {
  public static void main(String[] args) {
    
    int weeks = 3;
    int days = 7;

    // outer loop
    for(int i = 1; i <= weeks; ++i) {
      System.out.println("Week: " + i);

      // inner loop
      for(int j = 1; j <= days; ++j) {
        
        // continue inside the inner loop
        if(j % 2 != 0) {
          continue;
        }
        System.out.println("  Days: " + j);
      }
    }
  }
}

Output

Week: 1
  Days: 2
  Days: 4
  Days: 6
Week: 2
  Days: 2
  Days: 4
  Days: 6
Week: 3
  Days: 2
  Days: 4
  Days: 6

In the above example, we have used the continue statement inside the inner for loop. Notice the code,

if(j % 2 != 0) {
  continue;
}

Here, the continue statement is executed when the value of j is odd. Hence, the program only prints those days that are even.

We can see the continue statement has affected only the inner loop. The outer loop is working without any problem.