Print Butterfly Star Pattern in Java

Topic

In this program, we will learn how to print a butterfly star pattern using Java. The pattern has a mirror-like shape — the top half looks like a V and the bottom half is its reflection. It's a great example to practice nested loops and symmetry in pattern printing.

Examples

Input: n = 4 (number of rows for each half)

*      *
**    **
***  ***
********
********
***  ***
**    **
*      *

Each half of the butterfly is symmetrical. Spaces reduce as stars increase and vice versa.

Interviewer Expectations

The interviewer is checking your understanding of:

• How to use for loops, especially nested loops.
• How to print symmetrical shapes using patterns of spaces and stars.
• How well you structure your loops and maintain readable code.

Approach

We divide the butterfly pattern into two parts:

1. Top half: From row 1 to n
2. Bottom half: From row n to 1

Each row has two mirrored star sections, and between them are spaces. The number of stars is equal to the row number, and spaces are calculated as: 2 * (n - row).

Dry Run for n = 3 (Top Half)

Row 1: *    *   → 1 star + 4 spaces + 1 star
Row 2: **  **   → 2 stars + 2 spaces + 2 stars
Row 3: ******   → 3 stars + 0 spaces + 3 stars

Java Program

public class ButterflyPattern {
    public static void main(String[] args) {
        int n = 4;

        // Top half
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= i; j++)
                System.out.print("*");

            for (int j = 1; j <= 2 * (n - i); j++)
                System.out.print(" ");

            for (int j = 1; j <= i; j++)
                System.out.print("*");

            System.out.println();
        }

        // Bottom half
        for (int i = n; i >= 1; i--) {
            for (int j = 1; j <= i; j++)
                System.out.print("*");

            for (int j = 1; j <= 2 * (n - i); j++)
                System.out.print(" ");

            for (int j = 1; j <= i; j++)
                System.out.print("*");

            System.out.println();
        }
    }
}

*      *
**    **
***  ***
********
********
***  ***
**    **
*      *

Practice Butterfly Pattern

Possible Followup Questions with Answers

Q1: Can you print the same pattern with numbers instead of stars?

Yes, modify System.out.print("*") to System.out.print(i) or any logic for numbers.

Q2: How would you create a hollow butterfly pattern?

Add conditions inside star-printing loops to only print stars at the boundary (i.e., j == 1 || j == i), and spaces elsewhere.

Q3: How can we make the program dynamic for any n?

Take input from the user using Scanner and use that value in your loops.

Q4: What's the time complexity of this program?

The program runs in O(n²) time because each loop prints up to n characters across nested iterations.