2.1 Ballworld, inheritance-based  (Page 3/4)

ABall.updateState() is abstract because at the abstraction level of ABall , one only knows that the ball will definitely do something with regards to modifying (perhaps) its internal field values(its "state"). Each subclass will do it differently however. The StraightBall will do nothing in its updateState method because a ball with a constant (unchanging) velocity will travel in a straight line. Remember, doing nothing is doing something ! The CurveBall 's updateState method uses sines and cosines to turn the velocity by a fixed (though randomly chosen) angle at every update event. You can imagine that other possible subclassses could do things such as randomly change the velocity or change the radius of the ball or change the color of the ball.

There is no code in the entire Ballworld system that explicitly references any of the concrete ABall subclasses. All of the code runs at the level of abstraction of an abstract ball. The differences in behavior of the various balls made on the screen using the different concrete subclasses is strictly due to polymorphism . New types of balls can be added to the system without recompiling any of the existing code. In fact, new types of balls can be added without even stopping the Ballworld program!

Abstract classes vs. interfaces

Subclasses have a different relationship between interfaces and abstract superclasses. A subclass that implements an interface is saying simply that it "acts like" that specified by the interface. The class makes no statements however about fundamentally what it actually is. An actor implements a fiercesome alien from a distant plantet in one movie and a fickle feline in another. But an actor is actually neither. Just because the actor protrayed a interplanetary alien, doesn't mean that the actor fundamentally possessed all the abilities of such an alien. All it says is that in so far the context in which the actor was utilized as the alien, the actor did implement all the necessary behaviors of the alien.

A subclass is fundamentally an example of its superclass. A subclass automatically contains all the behaviors of its superclass because it fundamentally is the superclass. The subclass doesn't have to implement the behaviors of its superclass, it already has them. An actor is a human and by that right, automatically possesses all that which makes a human: one head, two arms, 10 toes, etc. Note that this is true even if the abstract class has 100% abstract methods--it still enforces a strict taxonomical hierarchy.

implements is about behaving , extends is about being .

Variant vs. invariant behaviors

A crucial observation is that the the Ballworld code that manages the GUI ( BallGUI ) and the ball management ( BallControl , Dispatcher , etc.) only deal with the abstract ball, ABall . That is, they represent invariant behavior at the abstract ball level. The display, creation and managment of the balls is independent of the particular kinds of concrete balls that is being handled. The main Ballworld framework can thus handle any type of ABall , past, present and future.