《SSD1 3课件》.ppt

SSD1: Introduction to Information Systems Unit 3: Inheritance 3.1 Introduction to Inheritance Introduction Terminology Inheritance in Action Advantages of Programming with Inheritance When Not to Use Inheritance Inheritance in Java Introduction Bank accounts can be further subdivided into groups such as savings accounts, and checking accounts. Defining much of the same code for both checking accounts and savings accounts. Using inheritance, an object acquires all the methods and attributes of the class from which it inherits. Terminology 被继承的类称为父类(parent class)，或称为超类(super class)、基类(base class)、超类型(supertype) 继承父类而产生的新类称为子类(subclass)、导出类(derived class)或子类型(subtype) Inheritance in Action It may seem very abstract at this point that a class can use the attributes and methods of another class without declaring them. CheckingAccount SavingsAccount Advantages of Programming with Inheritance First, inheritance allows programmers to design systems in an intuitive, in a top-down fashion. 编程者可以使用自上而下的模式设计系统，系统的设计清晰、直观 Second, it provides a framework for code re-use. 如：Catfish、Crocodile和Algae类中都需要使用一个die方法描述生物死去的行为，这三个生物死去的行为的实现都是一样的，使用了继承，就只需使用一次die()方法，而无需实现三次 我们能在LivingBeing类中定义AlgaeColony, Crocodile, and Catfish 共同的属性和方法. LivingBeing类能进一步在animals类和非animals类之间划分。 When Not to Use Inheritance Age, energy level, are alive (or not) should be generalized into the LivingBeing class Catfish、Crocodile和AlgaeColony都是LivingBeing的子类，这三类生物都有年龄(age)、能量(energy)以及是否活着(alive)属性，这些共同的属性可以放在LivingBeing It does not make sense to include the ability to move in the LivingBeing class since AlgaeColony cannot move. 但是Catfish和Crocodile可以移动，而水草不可以移动，因此移动行为的描述不能放在LivingBeing中实现 Inheritance in Java In Java, all classes are subclasses of Object and inherit all of Objects attributes and methods. We can design objects starting with their least specific forms, where all general behavior is shared in one place. We can then proceed to derive specific versions of these general objects. 3.2 U