Character Diamond Pattern

Java 8 has no String.repeat, so we use a small helper method. In Java 11 and newer you can write " ".repeat(count).

/*
 * Problem : Diamond of letters.
 * Input   : One integer n (rows in the top half).
 * Output  : 2n-1 lines.
 * Approach: Alphabet pyramid and its mirror.
 * Time    : O(n^2)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method returns the text repeated "count" times. Java 11 has String.repeat for this.
static String repeat(String text, int count) {
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < count; i++) {
        result.append(text);
    }
    return result.toString();
}

    // This method receives the half size and a row number. It returns one line, for example "  ABA".
    static String buildLine(int half, int row) {
        StringBuilder line = new StringBuilder(repeat(" ", half - row));
        for (int index = 0; index < row; index++) {
            line.append((char) ('A' + index));
        }
        for (int index = row - 2; index >= 0; index--) {
            line.append((char) ('A' + index));
        }
        return line.toString();
    }

    public static void main(String[] args) {
        int half = new Scanner(System.in).nextInt();

        // Step 1: Print the top half. Row numbers go from 1 up to half.
        for (int row = 1; row <= half; row++) {
            System.out.println(buildLine(half, row));
        }

        // Step 2: Print the bottom half. Start from half - 1 because the widest row is already printed.
        for (int row = half - 1; row >= 1; row--) {
            System.out.println(buildLine(half, row));
        }
    }
}