this.previousWrite = previous;
    }
  }

  /** References a weak value. */
  static class WeakValueReference<K, V> extends WeakReference<V> implements ValueReference<K, V> {
    final ReferenceEntry<K, V> entry;

    WeakValueReference(ReferenceQueue<V> queue, V referent, ReferenceEntry<K, V> entry) {
      super(referent, queue);
      this.entry = entry;
    }