code: @Nullable
      private DataType getCachedData(@NonNull final KeyType key) {
          synchronized (cache) {
              return cache.get(key);
          }
      }
docs: Returns the data, which corresponds to a specific key, from the cache.
  
  @param key
  The key of the data, which should be retrieved, as an instance of the generic type
  KeyType. The key may not be null
  @return The data, which has been retrieved, as an instance of the generic type DataType or
  null, if no data with the given key is contained by the cache