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original.java

@Beta
@Nullable
@CheckForNull
public static Long tryParse(String string) {
  if (checkNotNull(string).isEmpty()) {
    return null;
  }
  boolean negative = string.charAt(0) == '-';
  int index = negative ? 1 : 0;
  if (index == string.length()) {
    return null;
  }
  int digit = string.charAt(index++) - '0';
  if (digit < 0 || digit > 9) {
    return null;
  }
  long accum = -digit;
  while (index < string.length()) {
    digit = string.charAt(index++) - '0';
    if (digit < 0 || digit > 9 || accum < Long.MIN_VALUE / 10) {
      return null;
    }
    accum *= 10;
    if (accum < Long.MIN_VALUE + digit) {
      return null;
    }
    accum -= digit;
  }

  if (negative) {
    return accum;
  } else if (accum == Long.MIN_VALUE) {
    return null;
  } else {
    return -accum;
  }
}

modified.java

@Beta
@Nullable
@CheckForNull
public static Long tryParse(String string, int radix) {
  if (checkNotNull(string).isEmpty()) {
    return null;
  }
  if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX) {
    throw new IllegalArgumentException(
        "radix must be between MIN_RADIX and MAX_RADIX but was " + radix);
  }
  boolean negative = string.charAt(0) == '-';
  int index = negative ? 1 : 0;
  if (index == string.length()) {
    return null;
  }
  int digit = digit(string.charAt(index++));
  if (digit < 0 || digit >= radix) {
    return null;
  }
  long accum = -digit;

  long cap = Long.MIN_VALUE / radix;

  while (index < string.length()) {
    digit = digit(string.charAt(index++));
    if (digit < 0 || digit >= radix || accum < cap) {
      return null;
    }
    accum *= radix;
    if (accum < Long.MIN_VALUE + digit) {
      return null;
    }
    accum -= digit;
  }

  if (negative) {
    return accum;
  } else if (accum == Long.MIN_VALUE) {
    return null;
  } else {
    return -accum;
  }
}