Untitled

public class Diamond {

    static final String BASE = "ZYXWVUTSRQPONMLKJIHGFEDCBABCDEFGHIJKLMNOPQRSTUVWXYZ";

    public String generateDiamond(char c, char stop) {
        String unmaskedLine = BASE.substring(BASE.indexOf(stop), BASE.lastIndexOf(stop) + 1);
        String maskedLine = unmaskedLine.replaceAll("[^" + c + "]", " ") + "\n";

        return (c == stop) ? maskedLine : maskedLine + generateDiamond(++c, stop) + maskedLine;
    }

    public static void main(String[] args) {
        Diamond diamond = new Diamond();
        System.out.println(diamond.generateDiamond('A', 'R'));
    }
}