substitution

public class SubstitutionCipher {

public static String encrypt(String text, String key) {
    text = text.toUpperCase();
    StringBuilder result = new StringBuilder();
    for (char c : text.toCharArray()) {
        if (Character.isLetter(c)) {
            result.append(key.charAt(c - 'A'));
        } else {
            result.append(c);
        }
    }
    return result.toString();
}

public static String decrypt(String text, String key) {
    text = text.toUpperCase();
    StringBuilder result = new StringBuilder();
    for (char c : text.toCharArray()) {
        if (Character.isLetter(c)) {
            int index = key.indexOf(c);
            result.append((char)('A' + index));
        } else {
            result.append(c);
        }
    }
    return result.toString();
}

public static void main(String[] args) {
    String message = "HELLO WORLD";
    String key = "QWERTYUIOPASDFGHJKLZXCVBNM"; // substitution key

    String encrypted = encrypt(message, key);
    String decrypted = decrypt(encrypted, key);

    System.out.println("Substitution Cipher");
    System.out.println("Original : " + message);
    System.out.println("Encrypted: " + encrypted);
    System.out.println("Decrypted: " + decrypted);
}

}