Nested Loops in Java

The English meaning of nested is inside another. It means nested loops are the loop statement inside another loop statement. In simple terms, the loop within a loop is called a nested loop.

The inner loop runs fully before the outer loop moves to the next cycle. It is useful for handling repeated tasks, such as working with grids, printing patterns, or processing multi-layered data like schedules and tables.

In this section, we will discuss the types of nested loops in Java.

In Java, there are mainly following three types of nested loops:

1. Nested for Loop
2. Nested while Loop
3. Nested do-while Loop
4. Hybrid Nested Loop (combination of the above three)

Let’s discuss one by one.

Nested for Loop

A nested for loop is a for loop that is placed inside another for loop. The inner loop runs completely for each cycle of the outer loop before it moves to the next iteration.

Syntax:

Example of Nested for Loop

File Name: Main.java

Output:

 
Outer: 1, Inner: 1
Outer: 1, Inner: 2
Outer: 1, Inner: 3
Outer: 2, Inner: 1
Outer: 2, Inner: 2
Outer: 2, Inner: 3
Outer: 3, Inner: 1
Outer: 3, Inner: 2
Outer: 3, Inner: 3
Outer: 4, Inner: 1
Outer: 4, Inner: 2
Outer: 4, Inner: 3   

Use Cases of Nested for Loop

• Working with 2D arrays (like a chessboard).
• Printing patterns (stars, numbers, etc.).
• Processing tabular data (like spreadsheets).
• Handling combinations (pairing different choices).

Nested while Loop

A nested while loop is a while loop that is placed inside another while loop. The inner loop runs completely for each iteration of the outer loop before the outer loop moves to the next step.

Syntax:

Example of Nested while Loop

File Name: Main.java

Output:

 
Outer: 1, Inner: 1
Outer: 1, Inner: 2
Outer: 2, Inner: 1
Outer: 2, Inner: 2
Outer: 3, Inner: 1
Outer: 3, Inner: 2   

Use Cases of Nested while Loops

• Processing 2D data structures (like grids).
• Repeating tasks with unknown limits (when exact iterations aren’t known).
• Generating patterns dynamically.
• Handling user inputs in loops.

Nested do-while Loop

A nested do-while loop is a do-while loop that is placed inside another do-while loop. The inner loop runs completely for each cycle of the outer loop before moving to the next iteration of the outer loop. Do-while loops, in contrast to while and for loops, ensure that the loop will run at least once, even in the case in which the condition is false.

Syntax:

Example of Nested while Loop

File Name: Main.java

Output:

 
Outer: 1, Inner: 1
Outer: 1, Inner: 2
Outer: 2, Inner: 1
Outer: 2, Inner: 2
Outer: 3, Inner: 1
Outer: 3, Inner: 2   

Use Cases of Nested do-while Loop

• Ensuring a process runs at least once.
• Generating patterns dynamically when the number of iterations is not fixed.
• Working with user inputs that require at least one execution.
• Handling multi-dimensional data (like grids or tables).

Hybrid Nested Loops

It is not necessary that a loop must be nested inside its own type. We can put any type of loop inside a different type of loop. It is called a hybrid nested loop. For example, there may be while inside for loop, while inside do-while loop, for inside do-while loop, etc.

Syntax:

Example 1: while Loop Inside a for Loop

File Name: Main.java

Output:

 
Outer: 1, Inner: 1
Outer: 1, Inner: 2
Outer: 2, Inner: 1
Outer: 2, Inner: 2
Outer: 3, Inner: 1
Outer: 3, Inner: 2   

Example 2: do-while Loop Inside a while Loop

File Name: Main.java

Output:

 
Outer: 1, Inner: 1
Outer: 1, Inner: 2
Outer: 2, Inner: 1
Outer: 2, Inner: 2
Outer: 3, Inner: 1
Outer: 3, Inner: 2   

Use Cases of Hybrid-Nested Loop

• Handling different iteration requirements (for example, one loop runs at least once while another depends on a condition).
• Processing structured data like tables or grids.
• Generating dynamic patterns efficiently.

Advantages of Nested Loops

1. Good for Multi-Dimensional Data.
2. Good for Pattern Printing: Need to print triangles, pyramids, or checkerboard patterns? Nested loops provide precise control over rows and columns.
3. Useful for Sorting and Searching: Algorithms like Bubble Sort and Selection Sort use nested loops to compare elements and arrange them efficiently.
4. Solving Pairing and Combination Problems.
5. Simulating Real-World Scenarios.
6. Efficient for Repetitive Iterations.