private void moveToFront(LinkedEntry<T> bucket) {
    if (mHead == bucket) {
      return;
    }

    prune(bucket);

    if (mHead == null) {
      mHead = bucket;
      mTail = bucket;
      return;
    }

    bucket.next = mHead;
    mHead.prev = bucket;
    mHead = bucket;
  }
@return number of objects contained in the {@link BucketMap}
imagepipeline/src/main/java/com/facebook/imagepipeline/memory/BucketMap.java