code: public <T extends Serializer<?> & Serializable>void addDefaultKryoSerializer(Class<?> type, T serializer) {
  		if (type == null || serializer == null) {
  			throw new NullPointerException("Cannot register null class or serializer.");
  		}
  
  		defaultKryoSerializers.put(type, new SerializableSerializer<>(serializer));
  	}
docs: Adds a new Kryo default serializer to the Runtime.
  
  Note that the serializer instance must be serializable (as defined by java.io.Serializable),
  because it may be distributed to the worker nodes by java serialization.
  
  @param type The class of the types serialized with the given serializer.
  @param serializer The serializer to use.