Lecture 38

Announcements

Talking Points

Mutual Use Example

Often it is nice to have objects that refer to one another. For example, a Theater object might have many Movie objects it is showing, suggesting a UML class diagram like

Theater
movies : ArrayList<Movie>
addMovie(Movie) : void removeMovie(Movie) : void isShowing(Movie) : boolean

It also might make sense for a movie to know what theaters are showing it, suggesting a UML class diagram like

Movie
theaters : ArrayList<Theater>
addTheater(Theater) : void removeTheater(Theater) : void isAt(Theater) : boolean

Movie.java, Theater.java, and MovieTheaterTester.java

Data Consistency

When classes mutually refer to one another, we want to maintain consistent data: that is, we want t.isShowing(m) == m.isAt(t) to always be true for all Theater objects t and Movie objects m.

Data consistency has lots of potential problems, many related to networks:

Generally, film production companies and theaters are not the same person.
Basic pattern: Movie: please stop showing this movie. Theater: ok, it's off our marque. Movie: And I've unlisted you as a location too.
What happens if a Viewer comes along half-way through this conversation?
What happens if someone says no, not a good idea?
What happens if one of these messages doesn't make it to the other party?

Even without the network issues, there are still problems if we do not maintain consistency properly ourselves.