*   <li>time-based expiration of entries, measured since last access or last write
 *   <li>keys automatically wrapped in {@linkplain WeakReference weak} references
 *   <li>values automatically wrapped in {@linkplain WeakReference weak} or {@linkplain
 *       SoftReference soft} references
 *   <li>notification of evicted (or otherwise removed) entries
 *   <li>accumulation of cache access statistics

 *   <li>time-based expiration of entries, measured since last access or last write
 *   <li>keys automatically wrapped in {@linkplain WeakReference weak} references
 *   <li>values automatically wrapped in {@linkplain WeakReference weak} or {@linkplain
 *       SoftReference soft} references
 *   <li>notification of evicted (or otherwise removed) entries
 *   <li>accumulation of cache access statistics

    int iterations = 10;
    WeakReference<Object> ref = new WeakReference<>(null);
    int expectedComputations = 0;
    for (int i = 0; i < iterations; i++) {
      // The entry should get garbage collected and recomputed.
      Object oldValue = ref.get();
      if (oldValue == null) {
        expectedComputations++;
      }
      ref = new WeakReference<>(cache.getUnchecked(1));
      oldValue = null;

    int iterations = 10;
    WeakReference<Object> ref = new WeakReference<>(null);
    int expectedComputations = 0;
    for (int i = 0; i < iterations; i++) {
      // The entry should get garbage collected and recomputed.
      Object oldValue = ref.get();
      if (oldValue == null) {
        expectedComputations++;
      }
      ref = new WeakReference<>(cache.getUnchecked(1));
      oldValue = null;

      clearReferenceQueue(queueForKeys);
    }
  }

  static final class CleanupMapTask implements Runnable {
    final WeakReference<MapMakerInternalMap<?, ?, ?, ?>> mapReference;

    public CleanupMapTask(MapMakerInternalMap<?, ?, ?, ?> map) {
      this.mapReference = new WeakReference<>(map);
    }

    @Override
    public void run() {
      MapMakerInternalMap<?, ?, ?, ?> map = mapReference.get();