第2课第4节_Java面向对象编程_多态性_P【学习笔记】

摘要:韦东山android视频学习笔记 

面向对象程序的三大特性之继承性：

1、向上转换:只能定义被子类覆写的方法，不能调用在子类中定义的方法。

class Father {
    private int money;    

    public int getMoney() {return money; }
    public void setMoney(int money) {this.money = money; }

    public void printInfo() {System.out.println("This is Father");}
}

class Son extends Father{
    public void printInfo() {System.out.println("This is son");}
    public void playGame(){System.out.println("This is son");}
}

class Daughter extends Father{
    public void printInfo() {System.out.println("This is Father");}
}


public class Cnv {
    public static void main (String args[]) {
        Son son = new Son();
        Daughter daughter = new Daughter();

        Father f = son;
        f.printInfo();
        //f.playGame();

        f = daughter;
        f.printInfo();
    }
}

编译运行:

2、JAVA向下转换的例子,在进行对象的向下转换前，必须首先发生对象的向上转换.否则会编译不过.

class Father {
    private int money;    

    public int getMoney() {return money; }
    public void setMoney(int money) {this.money = money; }

    public void printInfo() {System.out.println("This is Father");}
}

class Son extends Father{
    public void printInfo() {System.out.println("This is son");}
    public void playGame(){System.out.println("This is son");}
}

class Daughter extends Father{
    public void printInfo() {System.out.println("This is Father");}
}


public class Cnv2 {
    public static void main (String args[]) {
        Father f = new Son();
        Son son = (Son)f;
    }
}

3、看一下，下面的例子，假如有一千个类继承了father这个类，如果我们要打印他们的信息，那样我们岂不是要写1千个print函数,下面的第二个代码则通过向上转换这个技巧实现.

class Father {
    private int money;    

    public int getMoney() {return money; }
    public void setMoney(int money) {this.money = money; }

    public void printInfo() {System.out.println("This is Father");}
}

class Son extends Father{
    public void printInfo() {System.out.println("This is son");}
    public void playGame(){System.out.println("This is son");}
}

class Daughter extends Father{
    public void printInfo() {System.out.println("This is Dauhter");}
}


public class Cnv3 {
    public static void main (String args[]) {
        Father f = new Father();
        Son s = new Son();
        Daughter d = new Daughter();

        print(f);
        print(s);
        print(d);
    }

    public static void print(Father f){
        f.printInfo();
    }

    
    public static void print(Son s){
            s.printInfo();
    }

    public static void print(Daughter d){
            d.printInfo();
    }


    
}

通过向上转换实现:

class Father {
    private int money;    

    public int getMoney() {return money; }
    public void setMoney(int money) {this.money = money; }

    public void printInfo() {System.out.println("This is Father");}
}

class Son extends Father{
    public void printInfo() {System.out.println("This is son");}
    public void playGame(){System.out.println("This is son");}
}

class Daughter extends Father{
    public void printInfo() {System.out.println("This is Dauhter");}
}


public class Cnv4 {
    public static void main (String args[]) {
        Father f = new Father();
        Son s = new Son();
        Daughter d = new Daughter();

        print(f);
        print(s);
        print(d);
    }
    

    public static void print(Father f){
        f.printInfo();
    }
}

上述两份代码编译运行结果一样:

4、instanceof: 用来判断一个对象是不是某个类的实例

class Father {
    private int money;    

    public int getMoney() {return money; }
    public void setMoney(int money) {this.money = money; }

    public void printInfo() {System.out.println("This is Father");}
    public void drink(){System.out.println("drink");}
}

class Son extends Father{
    public void printInfo() {System.out.println("This is son");}
    public void playGame(){System.out.println("playGame");}
}

class Daughter extends Father{
    public void printInfo() {System.out.println("This is Dauhter");}
    public void dance(){System.out.println("dance");}
}


public class Cnv5{
    public static void main (String args[]) {
        Father f = new Father();
        Son s = new Son();
        Daughter d = new Daughter();

        printAction(f);
        printAction(s);
        printAction(d);
    }
    

    public static void printAction(Father f){
        if (f instanceof Son){
            Son s = (Son)f;
            s.playGame();
        }else if (f instanceof Daughter){
            Daughter d = (Daughter)f;
            d.dance();
        }else if(f instanceof Father){
            f.drink();
        }
    }
}

编译运行结果:

相关代码存放在github，可以下载https://github.com/zzb2760715357/100ask