Inheritance

We now have a pretty good idea of what the PacMan class would look like, with methods such as move, chomp, and (maybe) jump, but what about the other actors in the game? Most notably, what about the Ghosts? If you have played Pac-Man, you consider the Ghosts to be the enemy. But if you’re thinking about how to program Pac-Man, you will notice that the Ghosts and PacMan are a lot alike. For instance, they can all move around the game board, but they can’t go through walls. In Chapter 8, “Functions and Methods,” you learned the importance of keeping your code DRY (don’t repeat yourself), but if we write a move method for the PacMan class and a move method for the Ghost class, we will be repeating ourselves. The answer to this problem is inheritance.

Inheritance means that a class can be a type of another class. As an example, I am from Nevada and am therefore a member of the Nevadan class. All members of the Nevadan class have certain attributes and behaviors that are specific to Nevadans (such as surviveRidiculousHeatWithLittleWater), but all members of the Nevadan class are also members of the American class. The Nevadan class (the subclass) inherits all the attributes and methods of the American class (the parent class). All Nevadans are Americans, but not all Americans are Nevadans. The Texan class, for instance, also inherits from the American class, but from not the Nevadan class. The concept of inheritance allows for code to be shared among different classes. Both the PacMan class and the Ghost class can inherit from a parent class that we’ll call MovableCharacter. The move method would be defined in the MovableCharacter class and shared by both the PacMan class and the Ghost class (see Figure 15.5).

Figure 15.5 Although they have their differences, Pac-Man and the Ghosts share a common parent (class), MovableCharacter.