public boolean isEmpty() {
    return size() == 0;
  }

  @Override
  public boolean containsKey(@Nullable Object key) {
    return get(key) != null;
  }

  @Override
  public boolean containsValue(@Nullable Object value) {
    return values().contains(value);
  }

  // Overriding to mark it Nullable
  @Override
  public abstract @Nullable V get(@Nullable Object key);

  /**

    for (Object key : lookupKeys) {
      assertTrue(cache.asMap().containsKey(key));
    }
    assertSame(extraValue, cache.asMap().get(extraKey));
    assertFalse(cache.asMap().containsValue(extraKey));
  }

  public void testBulkLoad_partial() throws ExecutionException {
    Object extraKey = new Object();
    Object extraValue = new Object();
    CacheLoader<Object, Object> loader =