IS-A and HAS-A

Using IS-A and HAS-A: Remember that when one class inherits from another, we say that the subclass extends the superclass.

When you want to know if one thing should extend another, apply the IS-A test.

Tub extends Bathroom, sounds reasonable. Until you apply the IS-A test.

To know if you’ve designed your types correctly, ask, “Does it make sense to say type X IS-A type Y?” If it doesn’t, you know there’s something wrong with the design, so if we apply the IS-A test, Tub IS-A Bathroom is definitely false.

What if we reverse it to Bathroom extends Tub? That still doesn’t work, Bathroom IS-A Tub doesn’t work. Tub and Bathroom are related, but not through inheritance. Tub and Bathroom are joined by a HAS-A relationship. Does it make sense to say “Bathroom HAS-A Tub”? If yes, then it means that Bathroom has a Tub instance variable.

In other words, Bathroom has a reference to a Tub, but Bathroom does not extend Tub and vice-versa.

Bathroom HAS-A Tub and Tub HAS-A Bubbles. But nobody inherits from (extends) anybody else.

The IS-A test works anywhere in the inheritance tree. If your inheritance tree is well-designed, the IS-A test should make sense when you ask any subclass if it IS-A any of its supertypes.

Remember, if X extends Y, X IS-A Y must make sense.

DO NOT use inheritance just so that you can reuse code from another class, if the relationship between the superclass and subclass violate either of the above two rules. For example, imagine you wrote special printing code in the Alarm class and now you need printing code in the Piano class, so you have Piano extend Alarm so that Piano inherits the printing code. That makes no sense! A Piano is not a more specific type of Alarm. (So the printing code should be in a Printer class, that all printable objects can take advantage of via a HAS-A relationship.)

Inheritance IS-A summary:

• A subclass extends a superclass.
• A subclass inherits all public instance variables and methods of the superclass, but does not inherit the private instance variables and methods of the superclass.
• Inherited methods can be overridden; instance variables cannot be overridden (although they can be redefined in the subclass, but that’s not the same thing, and there’s almost never a need to do it.)
• Use the IS-A test to verify that your inheritance hierarchy is valid. If X extends Y, then X IS-A Y must make sense.
• The IS-A relationship works in only one direction. A Hippo is an Animal, but not all Animals are Hippos.
• When a method is overridden in a subclass, and that method is invoked on an instance of the subclass, the overridden version of the method is called. (The lowest one wins.)
• If class B extends A, and C extends B, class B IS-A class A, and class C IS-A class B, and class C also IS-A class A.