@Nullable
  public synchronized T removeFromEnd() {
    LinkedEntry<T> last = mTail;
    if (last == null) {
      return null;
    }

    T value = last.value.pollLast();
    maybePrune(last);
    return value;
  }
@return number of objects contained in the {@link BucketMap}
imagepipeline/src/main/java/com/facebook/imagepipeline/memory/BucketMap.java