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Java Program to Print Diamond Outline Star Pattern

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In this program we print the outline of a diamond. Every row prints only its two edge stars, so the diamond is hollow.

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 a row width = 2 * row - 1 characters wide.
  • Inside a row, a star is printed only in the first and last column; every other position is a space.
  • Step 1 counts row up from 1 to numberOfRows, and Step 2 counts it back down from numberOfRows - 1 to 1, so the widest row is printed only once.
  • System.out.println() moves to the next line after each row.

3. Java Program

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

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

        // Step 2: bottom half, the same outline upside down, without repeating the middle row
        for (int row = numberOfRows - 1; row >= 1; row--) {
            int width = 2 * row - 1;
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= width; column++) {
                System.out.print(column == 1 || column == width ? "*" : " ");
            }
            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.

DiamondOutlineStarPatternInput.java 
import java.util.Scanner;

public class DiamondOutlineStarPatternInput {
    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: top half, the outline of a pyramid
        for (int row = 1; row <= numberOfRows; row++) {
            int width = 2 * row - 1;
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= width; column++) {
                System.out.print(column == 1 || column == width ? "*" : " ");
            }
            System.out.println();
        }

        // Step 2: bottom half, the same outline upside down, without repeating the middle row
        for (int row = numberOfRows - 1; row >= 1; row--) {
            int width = 2 * row - 1;
            for (int column = 1; column <= numberOfRows - row; column++) {
                System.out.print(" ");
            }
            for (int column = 1; column <= width; column++) {
                System.out.print(column == 1 || column == width ? "*" : " ");
            }
            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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