code: private static HashMap<String, MPApiResponse> getMapCache() {
          if (cache == null || cache.get() == null) {
              cache = new SoftReference(new HashMap<String, MPApiResponse>());
          }
          return cache.get();
      }
docs: Auxiliar method. It returns a Map with cached responses from the soft references variable.
  If the map does not exists, its instantiated and then returned.
  
  @return      HashMap object