Further abstraction techniques - Objects First with Java: A Practical Introduction Using BlueJ - page 352

To deal with more general actors, it seems like a good idea to introduce an Actor super-
class. The Actor class would serve as a superclass to all kinds of simulation participants,
independent of what they are. Figure 10.3 shows a class diagram for this part of the simu-
lation. The Actor and Animal classes are abstract, while Rabbit , Fox , and Hunter are
concrete classes.

Figure 10.3
Simulation structure
with Actor

Simulator

Actor

Animal Hunter

Rabbit Fox

The Actor class would include the common part of all actors. One thing all possible actors
have in common is that they perform some kind of action. We will also need to know whether
an actor is still active or not. So the only definitions in class Actor are those of abstract act
and isActive methods:

// all comments omitted

public abstract class Actor
{
abstract public void act(List<Actor> newActors);
abstract public boolean isActive();
}

This should be enough to rewrite the actor loop in the Simulator (Code 10.6), using class
Actor instead of class Animal . (Either the isAlive method could be renamed to isActive
or a separate isActive method in Animal could simply call the existing isAlive method.)

Exercise 10.52 Introduce the Actor class into your simulation. Rewrite the simulate-
OneStep method in Simulator to use Actor instead of Animal . You can do this even if
you have not introduced any new participant types. Does the Simulator class compile? Or is
there something else that is needed in the Actor class?