Java string get latest 4 char reverse loop

Question

I cannot figure how to obtain latest 4 char of string before zeroes

String str = "41f1f3d1f10000000000000000000000000000000000"

I want: d1f1

I've tried to revert string string than do straight loop

public static boolean checklast4digit(String risposta) {
    String crc = "";
    risposta = reverseIt(risposta);
    for (int i = 0; i < risposta.length(); i++) {
        if (risposta.charAt(i) != '0') crc = Character.toString(risposta.charAt(i + 3)) + Character.toString(risposta.charAt(i + 2)) + Character.toString(risposta.charAt(i + 1)) + Character.toString(risposta.charAt(i)); 
    }
    Log.i("Crc letto: ", crc);
    return true;
}

public static String reverseIt(String source) { //Inversione stringa
    int i, len = source.length();
    StringBuilder dest = new StringBuilder(len);

    for (i = (len - 1); i >= 0; i--) {
        dest.append(source.charAt(i));
    }

    return dest.toString();
}

Exception:

java.lang.StringIndexOutOfBoundsException

Answer 1

As mentioned in the comments, you are looping too far. If you want to access charAt(i+3) you should only loop until i < risposta.length() - 3

Also, you need to break out of your loop, once you have found your result:

for(int i=0 ;i < risposta.length() - 3 ;i++){
    if(risposta.charAt(i) != '0') {
        crc= Character.toString(risposta.charAt(i + 3)) + Character.toString(risposta.charAt(i+2)) + Character.toString(risposta.charAt(i+1)) + Character.toString(risposta.charAt(i));
        break;
    } 
}

Note that this only gives you a result, if you have 4 non-zero characters before the zeros.