code: public Channel getChannel(int channelId) {
          Channel channel = channels.putIfAbsent(channelId, new Channel(this, channelId));
          if (channel == null) {
              channel = channels.get(channelId);
          }
          return channel;
      }
docs: Return channel by id.
  
  @param channelId
  Channel id
  @return Channel by id