code: public static HintManager getInstance() {
          Preconditions.checkState(null == HINT_MANAGER_HOLDER.get(), "Hint has previous value, please clear first.");
          HintManager result = new HintManager();
          HINT_MANAGER_HOLDER.set(result);
          return result;
      }
docs: Get a new instance for {@code HintManager}.
  
  @return  {@code HintManager} instance