code: private void addMetadataProviderForMedia(String key, MetadataProvider provider) {
          if (!metadataProviders.containsKey(key)) {
              metadataProviders.put(key, Collections.newSetFromMap(new ConcurrentHashMap<MetadataProvider, Boolean>()));
          }
          Set<MetadataProvider> providers = metadataProviders.get(key);
          providers.add(provider);
      }
docs: Internal method that adds a metadata provider to the set associated with a particular hash key, creating the
  set if needed.
  
  @param key the hashKey identifying the media for which this provider can offer metadata (or the empty string if
  it can offer metadata for all media)
  @param provider the metadata provider to be added to the active set