设计原则——开闭原则

概念：一个软件实体如类、模块和函数应该对扩展开放，对修改关闭。不修改源码，通过新增类扩展功能。

例子： 描述车的属性，颜色，轮胎个数，现在加装一个导航仪（需要改动的地方）

interface ICar {    
     String getColor();
     
     int getWheels();
}

interface INewCar extends ICar {

     String getNavigation();

 }

class Car implements ICar {

    private String color;

    private int wheels;

     
    @Override
    public String getColor() {
        // TODO Auto-generated method stub
        return color;
    }

    @Override
    public int getWheels() {
        // TODO Auto-generated method stub
        return wheels;
    }
     
    public Car(String color, int wheels) {
        this.color = color;
        this.wheels = wheels;
    }

    @Override
    public String toString() {
        return "Car [color=" + color + ", wheels=" + wheels + "]";
    }

 }

class NewCar implements INewCar {

    private String color;

    private int wheels;
    
    private String navigation;

    public NewCar(String color, int wheels, String navigation) {
        this.color = color;
        this.wheels = wheels;
        this.navigation = navigation;
    } 
    
    @Override
    public String getColor() {
        // TODO Auto-generated method stub
        return color;
    }

    @Override
    public int getWheels() {
        // TODO Auto-generated method stub
        return wheels;
    }

    @Override
    public String getNavigation() {
        // TODO Auto-generated method stub
        return navigation;
    }

    @Override
    public String toString() {
        return "NewCar [color=" + color + ", wheels=" + wheels + ", navigation=" + navigation + "]";
    }
     
 }

测试

import java.util.ArrayList;

public class Test {
    
    private final static ArrayList<ICar> carList = new ArrayList<ICar>();
    
    static {
        carList.add(new Car("red", 4));
        carList.add(new Car("white", 2));
        carList.add(new NewCar("black", 4, "谷歌导航"));
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        for (ICar car : carList) {
//          car.toString();
            System.out.println(car.toString());
        }
    }
}

打印结果

Car [color=red, wheels=4]
Car [color=white, wheels=2]
NewCar [color=black, wheels=4, navigation=谷歌导航]

总结：开闭原则用扩展接口去实现，不改变源码，例子中直观的感受就是增加了新接口INewCar和新实现类NewCar.