## 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; } @Override public String getName() { // TODO Auto-generated method stub return this.name; } @Override public String getAuthor() { // TODO Auto-generated method stub return this.author; } @Override public int getPrice() { // TODO Auto-generated method stub return 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; } @Override public String getName() { // TODO Auto-generated method stub return this.name; } @Override public String getAuthor() { // TODO Auto-generated method stub return this.author; } @Override public int getPrice() { // TODO Auto-generated method stub return this.price; } } class OffNovelBooks extends NovelBooks{ public OffNovelBooks(String name, String author, int price) { super(name, author, price); // TODO Auto-generated constructor stub } @Override public int getPrice() { int rePrice = super.getPrice(); int nowPrice = 0; if(rePrice>4000){ nowPrice = rePrice*80/100; }else{ nowPrice = rePrice*90/100; } return nowPrice; } } ~~~