code: public static Callable<Object> callable(final PrivilegedAction<?> action) {
          if (action == null)
              throw new NullPointerException();
          return new Callable<Object>() {
              public Object call() { return action.run(); }};
      }
docs: Returns a {@link Callable} object that, when
  called, runs the given privileged action and returns its result.
  @param action the privileged action to run
  @return a callable object
  @throws NullPointerException if action null