6大设计原则(六)---开闭原则。

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英文名称：Open Closed Principle，OCP

定义：一个软件实体如类、模块和函数应该对扩展开发，对修改关闭。
大意：一个软件实体应该通过扩展来实现变化，而不是通过修改已有的代码来实现变化。

开闭原则就是java世界里最基础的设计原则。

软件实体包括：
* 项目或软件产品中按照一定的逻辑规则划分的模块
* 抽象和类
* 方法

一个实例：
* IBook定义了数据的三个属性：名称，作者，价格
* BookStore是书店
* NovelBook是一个具体的实现类

public class BookStore {public static List<NovelBooks> listBooks = new ArrayList<NovelBooks>();static{listBooks.add(new NovelBooks("三重门","韩寒",3000));listBooks.add(new NovelBooks("人生","路遥",2400));listBooks.add(new NovelBooks("荆棘鸟","考琳·麦卡洛",5000));listBooks.add(new NovelBooks("从你的全世界路过","张嘉佳",2500));}    public static void main(String[] args) {//返回默认的货币格式    NumberFormat formatter = NumberFormat.getCurrencyInstance();for(NovelBooks book: listBooks){System.out.println("书籍名称:"+book.getName()+"\t书籍作者:"+book.getAuthor()+"\t书籍价格："+formatter.format(book.getPrice()/100.0)+"元");}}}interface IBook{public String getName();public String getAuthor();public int getPrice();}class NovelBooks implements IBook{private int price;private String name;private String author;public NovelBooks(String name,String author,int price) {this.author = author;this.price =price;this.name = name;}@Overridepublic String getName() {// TODO Auto-generated method stubreturn this.name;}@Overridepublic String getAuthor() {// TODO Auto-generated method stubreturn this.author;}@Overridepublic int getPrice() {// TODO Auto-generated method stubreturn this.price;}}

需求更改：
当书店推出打折畅销，来获取更多的购买量。书店规定40元以下的书籍8折出售，50元以上的数据9折出售。
那么如果在NovelBooks类中修改getPrice方法就会导致代码修改量变大。
解决方案：
通过扩展实现变化。
遵守开闭原则，对软件实体进行扩展来达到改变需求的目的，而不是修改已经存在的代码。
定义一个打折类offNovelBooks继承自NovelBooks，覆写getPrice方法

public class BookStore {public static List<NovelBooks> listBooks = new ArrayList<NovelBooks>();static{listBooks.add(new OffNovelBooks("三重门","韩寒",3000));listBooks.add(new OffNovelBooks("人生","路遥",2400));listBooks.add(new OffNovelBooks("荆棘鸟","考琳·麦卡洛",5000));listBooks.add(new OffNovelBooks("从你的全世界路过","张嘉佳",2500));}    public static void main(String[] args) {    //返回默认的货币格式    NumberFormat formatter = NumberFormat.getCurrencyInstance();for(NovelBooks book: listBooks){System.out.println("书籍名称:"+book.getName()+"\t书籍作者:"+book.getAuthor()+"\t书籍价格："+formatter.format(book.getPrice()/100.0)+"元");}}}interface IBook{public String getName();public String getAuthor();public int getPrice();}class NovelBooks implements IBook{private int price;private String name;private String author;public NovelBooks(String name,String author,int price) {this.author = author;this.price =price;this.name = name;}@Overridepublic String getName() {// TODO Auto-generated method stubreturn this.name;}@Overridepublic String getAuthor() {// TODO Auto-generated method stubreturn this.author;}@Overridepublic int getPrice() {// TODO Auto-generated method stubreturn this.price;}}class OffNovelBooks extends NovelBooks{public OffNovelBooks(String name, String author, int price) {super(name, author, price);// TODO Auto-generated constructor stub}@Overridepublic int getPrice() {int rePrice = super.getPrice();int nowPrice = 0;if(rePrice>4000){nowPrice = rePrice*80/100;}else{nowPrice = rePrice*90/100;}return nowPrice;}}

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