StringToLong

public class StringToLong {
  public long stringToLong(String s) {
    return stringToLong(s, true, true, true);
  }

  public long stringToLong(
      String s,
      boolean allowHeadingTailingSpaces,  // allow spaces in the begin/end of string
      boolean allowNonDigit,        // allow non digit char in the end, will only parse all digits
      boolean allowSaturate        // will return Long.MAX_VALUE/Long.MIN_VALUE instead of throw exception
    ) {
    String str = s.trim();
    if (!allowHeadingTailingSpaces && str.length() != s.length() ) {
      throw new NumberFormatException(String.format("%s has heading/tailing spaces in the string", s));
    }

    final int N = str.length();
    long val = 0;
    boolean neg = false;
    int i = 0;
    if (i < N && str.charAt(0) == '-') {
      neg = true;
      i++;
    }
    for (; i < N; i++) {
      char c = str.charAt(i);
      if (!isDigit(c)) {
        if(!allowNonDigit) {
          throw new NumberFormatException(String.format("%s has non-digit character", s));
        } else {
          break;
        }
      }

      int charVal = ctoi(c);
      // check for overflow
      // long: -9223372036854775808 ~ 9223372036854775807
      if ((!neg && val >= 922337203685477580L && charVal > 7) ||
        ( neg && val <= -922337203685477580L && charVal > 8)) {
        if (!allowSaturate) {
          throw new NumberFormatException(String.format("%s is out of range", s));
        } else {
          val = (neg) ? Long.MIN_VALUE : Long.MAX_VALUE;
          break;
        }
      }
      val = 10 * val + ((neg) ? -charVal : charVal);
    }
    return val;
  }

  private boolean isDigit(char c) {
    return (c >= '0') && (c <= '9');
  }

  private int ctoi(char c) {
    return c - '0';
  }
}