Find A Substring

boolean contains(String string, String substring) {
    if (string == null || substring == null) throw new NullPointerException();
    int sublength = substring.length();
    if (sublength == 0) return true;
    int length = string.length();
    if (length < sublength) return false;
    char[] cs = string.toCharArray();
    char[] subcs = substring.toCharArray();
    for (int i = 0; i < length; ++i) {
        if (cs[i] == subcs[0]) {
            int j = 1;
            while (j < sublength && cs[i + j] == subcs[j]) ++j;
            if (j == sublength) return true;
        }
    }
    return false;
}