Print Hollow Diamond Star Pattern in Java

Topic

In this Java interview program, we’ll learn how to print a hollow diamond star pattern. The hollow diamond pattern consists of stars forming a diamond shape, but with the middle area empty except for the outline. This pattern is a common way to assess a beginner’s ability to use nested loops and conditionals in Java.

Examples

Input: n = 5 (number of rows in the upper half, total rows = 2 * n - 1)

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

Input: n = 3

  *  
 * * 
*   *
 * * 
  *  

Edge Case: n = 1

*

Interviewer Expectations

The interviewer is checking your understanding of:

• Nested loops (outer and inner loops)
• Conditional logic to print stars at correct positions
• Mathematical reasoning to build symmetrical shapes
• Clean code practices and variable naming

Approach

The diamond pattern can be divided into two parts: the top half and the bottom half. Each part has spaces and stars.

1. For each row, print leading spaces to align the stars symmetrically.
2. Print a star at the beginning and end of the row.
3. If it's not the first or last column in that row, print spaces in between to make it hollow.
4. Repeat the above for both the upper and lower half of the diamond.

Dry Run (n = 3)

// Top half
Row 1: 2 spaces + 1 star → "  *"
Row 2: 1 space + star + space + star → " * *"
Row 3: 0 spaces + star + 3 spaces + star → "*   *"

// Bottom half (excluding the middle)
Row 1: 1 space + star + space + star → " * *"
Row 2: 2 spaces + 1 star → "  *"

Java Program

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

        // Top half
        for (int i = 1; i <= n; i++) {
            for (int j = 1; j <= n - i; j++) {
                System.out.print(" ");
            }
            System.out.print("*");
            if (i != 1) {
                for (int j = 1; j <= (2 * i - 3); j++) {
                    System.out.print(" ");
                }
                System.out.print("*");
            }
            System.out.println();
        }

        // Bottom half
        for (int i = n - 1; i >= 1; i--) {
            for (int j = 1; j <= n - i; j++) {
                System.out.print(" ");
            }
            System.out.print("*");
            if (i != 1) {
                for (int j = 1; j <= (2 * i - 3); j++) {
                    System.out.print(" ");
                }
                System.out.print("*");
            }
            System.out.println();
        }
    }
}

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

Hollow Diamond Pattern

Possible Followup Questions with Answers

1. How do you print a filled diamond pattern instead of a hollow one?

Instead of printing spaces between the two stars, print stars continuously:

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

2. How can you print the pattern for an even number of rows?

The current logic works best for odd numbers to ensure symmetry. For even n, you can either:

• Reduce one row to make it odd
• Or adjust the logic to include two middle rows

3. How would you return the pattern as a string instead of printing?

Use a StringBuilder and append each line, then return the full string.

StringBuilder sb = new StringBuilder();
sb.append(" *\n");