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Java Program to Print Diamond Star Pattern

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In this program we print a diamond of stars. The top half is a pyramid that grows to nine stars, and the bottom half is the same pyramid upside down.

Like every pattern program, it is built from nested loops: the outer loop picks the row, and the inner loops print what goes on that row.

1. The Pattern

For 5 rows the program prints:

Pattern
    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

2. How the Program Works

Here is what each printed line is made of when numberOfRows is 5:

LineLeading spacesPrinted after the spaces
14*
23***
32*****
41*******
50*********
61*******
72*****
83***
94*
  • Both parts print numberOfRows - row spaces and then 2 * row - 1 stars, an odd number that changes by two from row to row.
  • Step 1 counts row up from 1 to numberOfRows, so the rows get wider.
  • Step 2 counts row down from numberOfRows - 1 to 1, so the rows get narrower again. It starts one row lower so the widest row is printed only once.
  • System.out.println() moves to the next line after each row.

3. Java Program

DiamondStarPattern.java 
public class DiamondStarPattern {
    public static void main(String[] args) {
        int numberOfRows = 5;

        // Step 1: the top half is a pyramid
        for (int row = 1; row <= numberOfRows; row++) {
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= 2 * row - 1; column++) {
                System.out.print("*");
            }
            System.out.println();
        }

        // Step 2: the bottom half is a downward pyramid, without repeating the middle row
        for (int row = numberOfRows - 1; row >= 1; row--) {
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= 2 * row - 1; column++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}
Output
    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

4. Print the Pattern for Any Number of Rows

To let the user choose the size of the pattern, read the number of rows with a Scanner. The loops stay the same.

DiamondStarPatternInput.java 
import java.util.Scanner;

public class DiamondStarPatternInput {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the number of rows: ");
        int numberOfRows = scanner.nextInt();
        scanner.close();

        // Step 1: the top half is a pyramid
        for (int row = 1; row <= numberOfRows; row++) {
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= 2 * row - 1; column++) {
                System.out.print("*");
            }
            System.out.println();
        }

        // Step 2: the bottom half is a downward pyramid, without repeating the middle row
        for (int row = numberOfRows - 1; row >= 1; row--) {
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= 2 * row - 1; column++) {
                System.out.print("*");
            }
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 4
   *
  ***
 *****
*******
 *****
  ***
   *

5. Summary

  • The outer loop picks the row; the inner loops print the spaces and the stars for that row.
  • Work out how many spaces and stars each row needs, then write one inner loop for each.
  • Keep numberOfRows in a variable, so the same program can print the pattern in any size.

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