7 - CMPSCI 187 Programming With Data Structures Lecture 7...

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CMPSCI 187: Programming With Data Structures Lecture 7: Java: Inheritance and Class Hierarchies 21 September 2011
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Java: Inheritance and Class Hierarchies • Classes Extending Classes: Subtypes and Supertypes • The Hierarchy of Classes: When is a Reference Legal? • Adding Interfaces to the Hierarchy • The Object Class: The toString and equals methods • Visibility: public , protected , and private • Inheritance in Project #2
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Classes Extending Classes: Subtypes, Supertypes • When we declare a class, we can have it extend another class, as with: public class Terrier extends Dog. • This makes Terrier a subclass of Dog, and Dog a superclass of Terrier. If CairnTerrier extends Terrier, it is also a subclass of Dog, and so forth. • Every Terrier object then is-a Dog object. Terriers have all the fields of Dogs and can run all their methods. They may also have their own fields and methods, declared in the Terrier declaration. If a Terrier method and a Dog method have the same name, the former overrides the latter. • As we discussed earlier, when we create a Terrier and use it in a Dog variable, it remains a Terrier and when we call an overridden method we get the Terrier version.
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The Hierarchy of Classes • We can draw a diagram (a graph ) where points ( nodes ) represent classes and we have a line ( edge ) from A down to B whenever B extends A.
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