synchronized (mutex) {
        return typePreservingCollection(delegate().get(key), mutex);
      }
    }

    @Override
    public boolean put(@ParametricNullness K key, @ParametricNullness V value) {
      synchronized (mutex) {
        return delegate().put(key, value);
      }
    }

    @Override
    public boolean putAll(@ParametricNullness K key, Iterable<? extends V> values) {