Java课件_LESSON 5 - Object-Oriented Programming 2.pptVIP

LESSON 5: Object-Oriented Programming 2 吴砥 wudi@ Department of Electronics Information Engineering, Huazhong University of Science Technology, Wuhan, P.R.C. Outline Inheritance Interface Abstract class Polymorphism Unified Modeling Language Summary Inheritance What is inheritance Inheritance is one of the most crucial concepts in object-oriented programming, and it has a very direct effect on how you design and write your Java classes. The benefits of inheritance Inheritance is a powerful mechanism that means when you write a class you only have to specify how that class is different from some other class, while also giving you dynamic access to the information contained in those other classes. With inheritance, all classes—those you write, those from other class libraries that you use, and those from the standard utility classes as well—are arranged in a strict hierarchy. Inheritance hierarchy The features of inheritance Subclasses inherit all the methods and variables from their superclasses—that is, in any particular class, if the superclass defines behavior that your class needs, you don’t have to redefine it or copy that code from some other class. In other word, the subclass becomes a combination of all the features of the classes above it in the hierarchy. The top super class At the top of the Java class hierarchy is the class Object; all classes inherit from this one superclass. Object is the most general class in the hierarchy; it defines behavior specific to all objects in the Java class hierarchy. The class diagram of Object Each class farther down in the hierarchy adds more information and becomes more tailored to a specific purpose. In fact, if you create a class definition that doesn’t indicate its superclass in the first line, Java automatically assumes you’re inheriting from Object. Subclassing Subclassing involves creating a new class that inherits from some other class in the class hierarchy. Using subclassing, you only need to define the di