1 基本介绍

开闭原则(OCP:Open Closed Principle)：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。用抽象构建架构，用实现扩展细节。当软件需要变化时，尽量通过扩展软件实体的行为来实现变化，而不是通过修改已有的代码来实现变化。

2 案例演示

public class Exer {
    public static void main(String[] args) {
        Coach coach = new Coach();
        coach.execPlay(new Runner());
        coach.execPlay(new Jumper());
    }
}
class Coach {
    public void execPlay(Sportsman man) {
        if (man.playType == 1) {
            execRun(man);
        } else if (man.playType == 2) {
            execJump(man);
        }
    }
    public void execRun(Sportsman man) {System.out.println("去跑步");}
    public void execJump(Sportsman man) {System.out.println("去跳");}
}
class Sportsman {
    int playType;
}
class Runner extends Sportsman {
    int playType = 1;
}
class Jumper extends Sportsman {
    int playType = 2;
}

分析：
① 优点：简单易操作
② 违反开闭原则，即对扩展开放，对修改关闭。即当我们给类增加新功能时，尽量不要修改代码，或者尽可能少修改代码。

当我新增功能时上面的代码就会需要改动很多地方，优化：

public class Exer {
    public static void main(String[] args) {
        Coach coach = new Coach();
        coach.execPlay(new Runner());
        coach.execPlay(new Jumper());
        coach.execPlay(new Swimer());
    }
}
class Coach {
    public void execPlay(Sportsman man) {
        man.execPlay();
    }
    // public void execRun(Sportsman man) {System.out.println("去跑步");}
    // public void execJump(Sportsman man) {System.out.println("去跳");}
}
abstract class Sportsman {
    int playType;
    public abstract void execPlay();
}
class Runner extends Sportsman {
    public Runner {super.playType = 1;}
    @Override
    public void execPlay() {System.out.println("去跑步");}
}
class Jumper extends Sportsman {
    public Jumper {super.playType = 2;}
    @Override
    public void execPlay() {System.out.println("去跳");}
}
// 新增功能
class Swimer extends Sportsman {
    public Swimer {super.playType = 3;}
    @Override
    public void execPlay() {System.out.println("去游泳");}
}