private void maybePrune(LinkedEntry<T> bucket) {
    if (bucket != null && bucket.value.isEmpty()) {
      prune(bucket);
      mMap.remove(bucket.key);
    }
  }
@return number of objects contained in the {@link BucketMap}
imagepipeline/src/main/java/com/facebook/imagepipeline/memory/BucketMap.java