code: public String substring(int beginIndex) {
          if (beginIndex < 0) {
              throw new StringIndexOutOfBoundsException(beginIndex);
          }
          int subLen = size() - beginIndex;
          if (subLen < 0) {
              throw new StringIndexOutOfBoundsException(subLen);
          }
          return (beginIndex == 0) ? toString() : new String(buf, toInternalId(beginIndex), subLen);
      }
docs: Wrapper of {@link String#substring(int)}
  
  @param beginIndex the begin index
  @return a String instance that is equivalent to a sub part of this FastStr