For the class diagram, let's imagine that we have a system that has to poll a queue for messages. Of course, this queue might not always have anything to offer, so it would return nulls. Instead of checking for null, we could simply return special null objects that will have empty behavior. Let's show these message classes in a diagram:
Using the classes from the preceding diagram, whenever there is no number to print, we will return a NullMessage object with empty behavior. In some cases, for optimization purposes, people could have the NullMessage as a singleton instance.
