Abstract Classes

• Guideline: Use abstract classes to provide default behavior or define a family.
• How do you choose between an abstract class and an interface?
• If you have default behavior, then you must use an abstract class
• Otherwise, use an interface unless you want to communicate limits
• Type Canine has one method, howl(), that is abstract
• If Canine is an abstract class, then expect subclasses like Dog, Wolf, etc...
• If Canine is an interface, then could get implementations like Blender

Figure 3-?. Inheritance and the state pattern.

It's really a tight-knit family thing. In the Java programming language, people can always use inner classes, which is a short hand of using composition, to get around multiple inheritance of implementation. If it feels more like a small family, go for the abstract class.

You force Canines into the Canine inheritance hierarchy. You limit client choices. What you are saying is that class Canine is a more narrow idea than interface Canine. If you use an abstract class, you are saying the concept is much more narrow, an interface, much more broad.