JAVA程序设计jhtp5_09.ppt

Chapter 9 - Object-Oriented Programming: Inheritance 9.1 Introduction Inheritance Software reusability Create new class from existing class Absorb existing class’s data and behaviors Enhance with new capabilities Subclass extends superclass Subclass More specialized group of objects Behaviors inherited from superclass Can customize Additional behaviors 9.1 Introduction Class hierarchy Direct superclass Inherited explicitly (one level up hierarchy) Indirect superclass Inherited two or more levels up hierarchy Single inheritance Inherits from one superclass Multiple inheritance Inherits from multiple superclasses Java does not support multiple inheritance 9.1 Introduction Abstraction Focus on commonalities among objects in system “is-a” vs. “has-a” “is-a” Inheritance subclass object treated as superclass object Example: Car is a vehicle Vehicle properties/behaviors also car properties/behaviors “has-a” Composition Object contains one or more objects of other classes as members Example: Car has a steering wheel 9.2 Superclasses and Subclasses Superclasses and subclasses Object of one class “is an” object of another class Example: Rectangle is quadrilateral. Class Rectangle inherits from class Quadrilateral Quadrilateral: superclass Rectangle: subclass Superclass typically represents larger set of objects than subclasses Example: superclass: Vehicle Cars, trucks, boats, bicycles, … subclass: Car Smaller, more-specific subset of vehicles 9.2 Superclasses and Subclasses (Cont.) Inheritance examples 9.2 Superclasses and Subclasses (Cont.) Inheritance hierarchy Inheritance relationships: tree-like hierarchy structure Each class becomes superclass Supply data/behaviors to other classes OR subclass Inherit data/behaviors from other classes 9.3 protected Members protected access Intermediate level of protection between public and private protected members accessible to superclass members subclass members Class members in the same package Subclass access s