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Head First Java, 2nd Edition by Bert Bates, Kathy Sierra

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Chapter 8. Interfaces and Abstract Classes: Serious Polymorphism

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Inheritance is just the beginning. To exploit polymorphism, we need interfaces (and not the GUI kind). We need to go beyond simple inheritance to a level of flexibility and extensibility you can get only by designing and coding to interface specifications. Some of the coolest parts of Java wouldn’t even be possible without interfaces, so even if you don’t design with them yourself, you still have to use them. But you’ll want to design with them. You’ll need to design with them. You’ll wonder how you ever lived without them. What’s an interface? It’s a 100% abstract class. What’s an abstract class? It’s a class that can’t be instantiated. What’s that good for? You’ll see in just a few moments. But if you think about the end of the last chapter, and how we used polymorphic arguments so that a single Vet method could take Animal subclasses of all types, well, that was just scratching the surface. Interfaces are the poly in polymorphism. The ab in abstract. The caffeine in Java.

Did we forget about something when we designed this?

The class structure isn’t too bad. We’ve designed it so that duplicate code is kept to a minimum, and we’ve overridden the methods that we think should have subclass-specific implementations. We’ve made it nice and flexible from a polymorphic perspective, because we can design Animal-using programs ...

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