@Nullable
  public synchronized T acquire(int key) {
    LinkedEntry<T> bucket = mMap.get(key);
    if (bucket == null) {
      return null;
    }
    T result = bucket.value.pollFirst();
    moveToFront(bucket);

    return result;
  }
@param key
@return Retrieve an object that corresponds to the specified {@code key} if present in the
    {@link BucketMap} or null otherwise
imagepipeline/src/main/java/com/facebook/imagepipeline/memory/BucketMap.java