Print Sandglass Star Pattern in Java

Topic: Print Sandglass Star Pattern in Java

The sandglass star pattern resembles an hourglass or sand timer. It consists of a decreasing number of stars printed in a centered format followed by an increasing number of stars, also centered. This creates a symmetric shape around the middle line.

In Java, this pattern helps beginners understand nested loops, space and star printing control, and how to mirror patterns.

Examples

Input: 5

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

Input: 3

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

Edge Case (Input = 1):

*

Interviewer Expectations

The interviewer is checking your ability to:

• Understand nested loop structures
• Control spaces and star counts symmetrically
• Write clean, readable code with proper formatting
• Break down the problem into two mirrored parts

Approach

We split the sandglass into two halves:

• Upper half: Print decreasing stars (2*n - 1, 2*n - 3, ..., 1) with increasing leading spaces.
• Lower half: Print increasing stars (3, 5, ..., 2*n - 1) with decreasing leading spaces.

Dry Run (n = 3)

Upper half:

• Row 0: 0 spaces, 5 stars → *****
• Row 1: 1 space, 3 stars → ***
• Row 2: 2 spaces, 1 star → *

Lower half:

• Row 1: 1 space, 3 stars → ***
• Row 2: 0 spaces, 5 stars → *****

Java Program

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

        // Upper half
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < i; j++) {
                System.out.print(" ");
            }
            for (int j = 0; j < 2 * (n - i) - 1; j++) {
                System.out.print("*");
            }
            System.out.println();
        }

        // Lower half
        for (int i = 1; i < n; i++) {
            for (int j = 0; j < n - i - 1; j++) {
                System.out.print(" ");
            }
            for (int j = 0; j < 2 * i + 1; j++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

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

Practice Sandglass Pattern

Possible Followup Questions with Answers

1. How to make the pattern hollow?

Use conditions to print "*" only at the first and last star positions per line, and print spaces in between.

if (j == 0 || j == totalStars - 1)
    System.out.print("*");
else
    System.out.print(" ");

2. How to take the height from user input?

Scanner sc = new Scanner(System.in);
int n = sc.nextInt();

3. What happens if we start both loops from 0?

You will duplicate the middle line. We use i = 1 in the lower half to avoid repeating the center of the hourglass.

4. Can this be done with a single loop?

Yes, but it complicates logic and decreases readability. Two separate loops are better for clarity.