code: public static <T> T getDone(Future<T> future)
      {
          requireNonNull(future, "future is null");
          checkArgument(future.isDone(), "future not done yet");
          return getFutureValue(future);
      }
docs: Returns the result of the input {@link Future}, which must have already completed.
  <p>
  Similar to Futures{@link Futures#getDone(Future)}, but does not throw checked exceptions.