code: public synchronized void addHandler(final short packetID, final PacketHandler packetHandler)
      {
          if (registry.containsKey(packetID)) throw new IllegalArgumentException("Handler for ID: " + packetID + " already exists");
          registry.put(packetID, packetHandler);
      }
docs: Adds a new handler for this specific Packet ID
  @param packetID Packet ID to add handler for
  @param packetHandler Handler for given Packet ID
  @throws IllegalArgumentException when Packet ID already has a registered handler