code: public static Callable<Object> callable(Runnable task) {
          if (task == null)
              throw new NullPointerException();
          return new RunnableAdapter<Object>(task, null);
      }
docs: Returns a {@link Callable} object that, when
  called, runs the given task and returns {@code null}.
  @param task the task to run
  @return a callable object
  @throws NullPointerException if task null