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Print Butterfly Pattern in Java

advanced· Patterns · Star Patterns

Problem

A butterfly pattern is two triangles of stars facing each other, widest at the top and bottom and touching in the middle row, formed by combining a growing gap with a shrinking one across two mirrored halves.

Given a size n, print a butterfly-shaped star pattern with two n-row wings that meet in the middle.

Input

5

Output

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

Java Program

Java

public class ButterflyPattern {
    public static void main(String[] args) {
        int n = 5;

        for (int i = 1; i <= n; i++) {
            StringBuilder line = new StringBuilder();
            for (int j = 1; j <= i; j++) line.append('*');
            for (int j = 1; j <= 2 * (n - i); j++) line.append(' '); // shrinking gap between the wings
            for (int j = 1; j <= i; j++) line.append('*');
            System.out.println(line);
        }

        for (int i = n - 1; i >= 1; i--) {
            StringBuilder line = new StringBuilder();
            for (int j = 1; j <= i; j++) line.append('*');
            for (int j = 1; j <= 2 * (n - i); j++) line.append(' ');
            for (int j = 1; j <= i; j++) line.append('*');
            System.out.println(line);
        }
    }
}

▶ Run Code

Output

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

Core Logic

Each row is built from three pieces — a left wing of stars, a gap that shrinks as the wings grow, and a matching right wing — and running that same row-building logic forward then backward produces the full butterfly.

How It Works

1The top half runs i from 1 to n: i stars, then 2 * (n - i) spaces, then i more stars.
2As i grows, the wings get wider and the middle gap shrinks, until i == n closes the gap entirely and the row becomes solid stars.
3The bottom half runs the exact same row-building logic with i from n - 1 back down to 1, mirroring the top half.
4Reusing one row-building shape for both halves is what keeps the pattern symmetric — nothing about the top or bottom half is computed differently.

For n = 5, row 3 has 3 stars, a gap of 2 * (5 - 3) = 4 spaces, then 3 more stars; row 5, the middle, has a gap of 0 and reads as ten solid stars.

💡

Key Point: The gap size 2 * (n - i) is what makes the wings meet exactly at the middle row — at i = n the gap hits zero, and any other formula would leave a visible seam or overlap.

Complexity

Time Complexity: O(n²)Space Complexity: O(1)

Why: Every row builds a line whose length is proportional to n, across roughly 2n rows total, so the total characters printed scale with n².

Print Butterfly Pattern in Java

Problem

Java Program

Output

Core Logic

Key Concepts

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