Nested Loops

Nested loops in Java are loops inside another loop. They are used when a task requires repeated execution within repeated execution, such as matrix processing, pattern printing, and multi-dimensional data handling. This concept is essential for logic building and interviews.

What Are Nested Loops?

• A loop placed inside another loop
• Inner loop executes completely for each outer loop iteration
• Can be any combination of for, while, or do-while

Basic Structure

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        System.out.println(i + "," + j);
    }
}
          

Execution Flow (Important)

• Outer loop runs first
• For each iteration of outer loop, inner loop runs fully
• Total iterations = (outer loop count × inner loop count)

Example with Execution Count

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        System.out.print("* ");
    }
    System.out.println();
}
          

Output:

* *
* *
* *
          

Total executions: Outer = 3, Inner = 2 → 6 executions

Nested for Loop (Most Common)

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.print(i + j + " ");
    }
    System.out.println();
}
          

Nested while Loop

int i = 1;
while (i <= 3) {
    int j = 1;
    while (j <= 2) {
        System.out.println(i + "," + j);
        j++;
    }
    i++;
}
          

Mixed Nested Loops

for (int i = 1; i <= 2; i++) {
    int j = 1;
    while (j <= 3) {
        System.out.println(i + "," + j);
        j++;
    }
}
          

Nested Loops with break

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            break;
        }
        System.out.println(i + "," + j);
    }
}
          

Nested Loops with Labeled break

outer:
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (i == 2 && j == 2) {
            break outer;
        }
        System.out.println(i + "," + j);
    }
}
          

Nested Loops with continue

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            continue;
        }
        System.out.println(i + "," + j);
    }
}
          

Common Use Cases

• Pattern printing
• Matrix operations
• Tables and grids
• Searching in 2D data
• Combinational logic

Performance Consideration (Important)

• Time complexity increases with nesting
• Example: O(n²) for two nested loops

Best Practice: Avoid unnecessary nesting in large datasets.

Common Beginner Mistakes

• Infinite loops due to incorrect inner loop update
• Confusing loop variables
• Excessive nesting reducing readability
• Forgetting braces {}

Interview-Ready Answers

Short Answer

Nested loops are loops placed inside another loop, where the inner loop executes fully for each iteration of the outer loop.

Detailed Answer

In Java, nested loops allow repetitive processing within repeated execution. They are commonly used for matrix handling, pattern generation, and multi-dimensional data processing. However, excessive nesting can impact performance.

Key Takeaway

Nested loops are powerful for multi-level iteration, but should be used carefully to maintain readability and performance.