public V waitForValue() {
      return get();
    }
  }

  /** References a soft value. */
  private static class SoftValueReference<K, V> extends SoftReference<V>
      implements ValueReference<K, V> {
    final ReferenceEntry<K, V> entry;

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