NibbleMonster.java

import java.util.Scanner;

public class NibbleMonster {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        while (true) {
            System.out.print("Feed the monster a nibble (hex digit): ");
            String input = scanner.nextLine();
            
            if (input.length() == 1) {
                char nibble = input.charAt(0);
                
                if (isHexDigit(nibble)) {
                    System.out.println(getHappyMessage(nibble));
                } else {
                    System.out.println("Monster is unhappy! That's not a nibble.");
                }
            } else {
                System.out.println("Monster is unhappy! Please enter only one character.");
            }
            scanner.close();
        }
    }
    
    public static boolean isHexDigit(char c) {
        return Character.isDigit(c) || (c >= 'A' && c <= 'F') || (c >= 'a' && c <= 'f');
    }
    
    public static String getHappyMessage(char nibble) {
        if (Character.isDigit(nibble) && nibble >= '0' && nibble <= '9') {
            return "Monster is happy! It loves " + nibble;
        } else if ((nibble >= 'A' && nibble <= 'F')) {
            return "Monster is wants more! It loves " + nibble + " (uppercase)";
        } else if ((nibble >= 'a' && nibble <= 'f')) {
            return "Monster is nibbling " + nibble + " (lowercase)";
        } else {
            return "Monster is unhappy!";
        }
    }
}