public class part3 {
    public static void main(String[] args) {
        String decryptKey = "QWERTYUIOPASDFGHJKLZXCVBNM";
        String decryptKey2 = "qwertyuiopasdfghjklzxcvbnm";
        String msg = "Hn fig vky dckm ephl, fig pkwc oie cwcdfephyo bidrhyo.\n" +
                     "Bcss miyc hymccm!\n" +
                     "Figd jdiodkz pkm lgvvcllngssf mcvdfjecm epc zcllkoc.\n";
        int i;
        int index = 0;
        int index2 = 0;

        for (i = 0; i < msg.length(); i++) {
            char c = msg.charAt(i);
                if (Character.isUpperCase(c)) {
                    char charDecrypt = Character.toUpperCase(c);
                    index = charDecrypt - 'A';
                    System.out.print(decryptKey.charAt(index));
                }
                else if (Character.isLowerCase(c) ) {
                    char charDecrypt2 = Character.toLowerCase(c);
                    index2 = charDecrypt2 - 'a';
                    System.out.print(decryptKey2.charAt(index2));
                }
                else {
                    System.out.print(c);
                }
        }
    }
}

// Output: If you can read this, you have got everything working.
Well done indeed!
Your program had successfully decrypted the message.