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Java Program to Print Diamond Character Pattern

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In this program we print a diamond of letters. The top half is a pyramid that grows from A to A B C D E, 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. Java stores a char as a number, so adding to 'A' and casting back with (char) gives the following letters.

1. The Pattern

For 5 rows the program prints:

Pattern
    A
   A B
  A B C
 A B C D
A B C D E
 A B C D
  A B C
   A B
    A

2. How the Program Works

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

LineLeading spacesPrinted after the spaces
14A
23A B
32A B C
41A B C D
50A B C D E
61A B C D
72A B C
83A B
94A
  • Both parts print numberOfRows - row spaces and then row letters, starting at A, each followed by a space.
  • 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

Pattern16.java 
public class Pattern16 {
    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 <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            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 <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            System.out.println();
        }
    }
}
Output
    A
   A B
  A B C
 A B C D
A B C D E
 A B C D
  A B C
   A B
    A

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.

Pattern16Input.java 
import java.util.Scanner;

public class Pattern16Input {
    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 <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            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 <= row; column++) {
                System.out.print((char) ('A' + column - 1) + " ");
            }
            System.out.println();
        }
    }
}
Output (with sample input)
Enter the number of rows: 4
   A
  A B
 A B C
A B C D
 A B C
  A B
   A
Note: there are only 26 letters. Past Z, (char) ('A' + 26) is [, not a letter, so keep the number of rows small enough that the pattern never needs a letter after Z.

5. Summary

  • The outer loop picks the row; the inner loops print the spaces and the letters for that row.
  • (char) ('A' + k) gives the letter k places after A.
  • Keep numberOfRows in a variable, so the same program can print the pattern in any size.

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