for (int i = 0; i < len; i++) { char ch = charAt(i);
int i = 0; char ch = charAt(0);  if (ch == '-' || ch == '+') i++;  for (; i < len; i++) { ch = charAt(i);
  public boolean isLongConvertible()  {    int len = length();    if (len == 0)      return false;    for (int i = 0; i < len; i++) {      char ch = charAt(i);      if (! ('0' <= ch && ch <= '9'))        return false;    }    return true;  }