return typePreservingCollection(entry.getValue(), mutex);
            }
          };
        }
      };
    }

    // See Collections.CheckedMap.CheckedEntrySet for details on attacks.

    @Override
    public @Nullable Object[] toArray() {
      synchronized (mutex) {
        /*
         * toArrayImpl returns `@Nullable Object[]` rather than `Object[]` but only because it can

   * the warnings the {@link MoreExecutors#directExecutor} documentation.
   *
   * <p>For a more general interface to attach a completion listener to a {@code Future}, see {@link
   * ListenableFuture#addListener addListener}.
   *
   * @param future The future attach the callback to.
   * @param callback The callback to invoke when {@code future} is completed.