@VisibleForTesting
  synchronized int valueCount() {
    int count = 0;
    LinkedEntry entry = mHead;
    while (entry != null) {
      if (entry.value != null) {
        count += entry.value.size();
      }
      entry = entry.next;
    }
    return count;
  }
@return number of objects contained in the {@link BucketMap}
imagepipeline/src/main/java/com/facebook/imagepipeline/memory/BucketMap.java