code: @Override
      public void addConverter(Class<?> from, Class<?> to, Converter converter) {
          Map toMap = fromMap.get(from);
          if (toMap == null) {
              toMap = new HashMap<Class<?>, Map<Class<?>, Converter>>();
              fromMap.put(from, toMap);
          }
          toMap.put(to, converter);
      }
docs: Converter calls this method to register conversion path.
  
  @param from
  @param to
  @param converter