* @deprecated Unsupported operation.
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final boolean retainAll(Collection<?> elementsToKeep) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the collection unmodified.
   *

        QueryResponseList qrList = new QueryResponseList(documentList, 0, 10, 0);

        // Test removeAll and retainAll with empty collection
        assertFalse(qrList.removeAll(new ArrayList<>()));
        assertEquals(1, qrList.size());

        assertTrue(qrList.retainAll(new ArrayList<>()));
        assertEquals(0, qrList.size());
    }

    @Test

   *
   * @throws UnsupportedOperationException always
   * @deprecated Unsupported operation.
   */
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")
  public final boolean retainAll(Collection<?> elementsToKeep) {
    throw new UnsupportedOperationException();
  }

  /**
   * Guaranteed to throw an exception and leave the collection unmodified.
   *

      assertThat(keys.remove(6)).isFalse();
      assertThat(keys.remove(-6)).isFalse();
      assertThat(keys.removeAll(asList(null, 0, 15, 1500))).isFalse();
      assertThat(keys.retainAll(asList(null, 0, 15, 1500))).isFalse();
      checkEmpty(keys);
      checkEmpty(cache);
    }
  }

  public void testKeySet_remove() {
    for (LoadingCache<Object, Object> cache : caches()) {

    // Split out all the specified sizes.
    Set<Feature<?>> sizesToTest = Helpers.copyToSet(CollectionSize.values());
    sizesToTest.retainAll(features);
    features.removeAll(sizesToTest);

    addImpliedFeatures(sizesToTest);
    sizesToTest.retainAll(asList(CollectionSize.ZERO, CollectionSize.ONE, CollectionSize.SEVERAL));

    logger.fine("   Sizes: " + formatFeatureSet(sizesToTest));

      boolean didRemove = entrySet.retainAll(entriesToRetain);
      assertEquals(shouldRemove, didRemove);
      assertEquals(entriesToRetain.size(), map.size());
      for (Entry<K, V> entry : entriesToRetain) {
        assertTrue(entrySet.contains(entry));
      }
    } else {
      assertThrows(UnsupportedOperationException.class, () -> entrySet.retainAll(entriesToRetain));
    }
    assertInvariants(map);

        removeIfEmpty();
      }
      return changed;
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      checkNotNull(c);
      int oldSize = size(); // calls refreshIfEmpty
      boolean changed = delegate.retainAll(c);
      if (changed) {
        int newSize = delegate.size();
        totalSize += newSize - oldSize;
        removeIfEmpty();
      }

  }

  public void testRetainAll_collection() {
    List<String> list = newArrayList("a", "b", "c", "d", "e");
    assertTrue(Iterables.retainAll(list, newArrayList("b", "d", "f")));
    assertEquals(newArrayList("b", "d"), list);
    assertFalse(Iterables.retainAll(list, newArrayList("b", "e", "d")));
    assertEquals(newArrayList("b", "d"), list);
  }

  public void testRetainAll_iterable() {

   */
  @Deprecated
  public static <T> Set<T> intersection(Set<? extends T> set1, Set<? extends T> set2) {
    Set<T> result = copyToSet(set1);
    result.retainAll(set2);
    return result;
  }

  private FeatureUtil() {}

   */
  @Deprecated
  public static <T> Set<T> intersection(Set<? extends T> set1, Set<? extends T> set2) {
    Set<T> result = copyToSet(set1);
    result.retainAll(set2);
    return result;
  }

  private FeatureUtil() {}