6大设计原则(六)—开闭原则。
最后更新于：2022-04-01 16:25:51
## 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;
}
}
~~~