}

  private static <T> AsyncCallable<T> constantAsyncCallable(
      final @Nullable ListenableFuture<T> returnValue) {
    return new AsyncCallable<T>() {
      @Override
      public ListenableFuture<T> call() {
        return returnValue;
      }
    };
  }

  /** Runnable which can be called a single time, and only after {@link #expectCall} is called. */

  }

  private static <T> AsyncCallable<T> constantAsyncCallable(
      final @Nullable ListenableFuture<T> returnValue) {
    return new AsyncCallable<T>() {
      @Override
      public ListenableFuture<T> call() {
        return returnValue;
      }
    };
  }

  /** Runnable which can be called a single time, and only after {@link #expectCall} is called. */