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Java Program to Print Diamond Number Pattern

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In this program we print a diamond of numbers. The top half is a pyramid that grows from 1 to 123456789, 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
    1
   123
  12345
 1234567
123456789
 1234567
  12345
   123
    1

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
141
23123
3212345
411234567
50123456789
611234567
7212345
83123
941
  • The program has two parts. Both print numberOfRows - row spaces and then the numbers from 1 to 2 * row - 1.
  • Step 1 counts row up from 1 to numberOfRows, so the rows get longer.
  • Step 2 counts row down from numberOfRows - 1 to 1, so the rows get shorter 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

Pattern17.java 
public class Pattern17 {
    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(column);
            }
            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(column);
            }
            System.out.println();
        }
    }
}
Output
    1
   123
  12345
 1234567
123456789
 1234567
  12345
   123
    1

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.

Pattern17Input.java 
import java.util.Scanner;

public class Pattern17Input {
    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(column);
            }
            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(column);
            }
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 4
   1
  123
 12345
1234567
 12345
  123
   1
Note: once the pattern needs numbers above 9, two-digit numbers take up two positions and the shape is no longer lined up. Print each number with a fixed width, for example System.out.printf("%3d", value), and print three spaces wherever the program prints one, to keep it lined up.

5. Summary

  • The outer loop picks the row; the inner loops print the spaces and the numbers for that row.
  • Work out how many spaces and which numbers 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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