Java学习笔记70——多态中的转型问题

多态中的转型问题

猫狗案例练习多态版

class Animal{
    public void eat(){
        System.out.println("吃饭");
    }
}

class Dog extends Animal{
    @Override
    public void eat() {
        System.out.println("狗吃肉");
    }

    public void lookDoor(){
        System.out.println("看门");
    }
}

class Cat extends Animal{
    @Override
    public void eat() {
        System.out.println("猫吃鱼");
    }
    public void catchMouse(){
        System.out.println("猫抓老鼠");
    }
}


public class PolymorphicDemo3 {
    public static void main(String[] args) {

        //多态的形式创建一个对象
        Animal a =new Dog();
        a.eat();

        //向下转型访问子类特有的方法
        Dog d=(Dog)a;
        d.eat();
        d.lookDoor();

        //java.lang.ClassCastException  类型转换异常
        //Cat c = (Cat)a; //报错。此刻的内存中还是一个Dog对象
        //c.eat();
        //c.catchMouse();

    }
}

多态中类型转换异常图示

class Animal2{
    public void eat(){
        System.out.println("吃饭");
    }
}

class Dog2 extends Animal2{
    @Override
    public void eat() {
        System.out.println("狗吃肉");
    }

    public void lookDoor(){
        System.out.println("狗看门");
    }

}

class Cat2 extends Animal2{
    @Override
    public void eat() {
        System.out.println("猫吃鱼");
    }

    public void catchMouse(){
        System.out.println("猫抓老鼠");
    }
}

public class PolymorphicTest1 {
    public static void main(String[] args) {
        Animal2 a = new Dog2();
        a.eat();
//        a.lookDoor();
        System.out.println("-----------------");

        Dog2 d = (Dog2)a;
        d.eat();
        d.lookDoor();
        System.out.println("------------------");
        a = new Cat2();
        a.eat();
//        a.catchMouse();
        System.out.println("------------------");
        Cat2 c = (Cat2)a;
        c.eat();
        c.catchMouse();
//        c.lookDoor();

        System.out.println("------------------");
//        Dog2 dog2 = new Animal2();//报错。子类引用指向父类，反了

//        Dog2 dog2 = new Cat2();//报错。两个类之间不存在继承关系

//        Dog2 dog2 = (Dog2)a;//报错。Dog类地址值在内存中无法赋值给Cat地址值
//        dog2.eat();


    }
}

不同地方饮食文化不同的案例

class Person{
    public void eat(){
        System.out.println("吃饭");
    }
}

class SouthPerson extends Person{
    @Override
    public void eat() {
        System.out.println("吃米饭");
    }

    public void work(){
        System.out.println("创业");
    }
}

class NorthPerson extends Person{
    @Override
    public void eat() {
        System.out.println("吃面食");
    }

    public void bath(){
        System.out.println("搓澡");
    }
}


public class PolymorphinTest {
    public static void main(String[] args) {

        Person p =new SouthPerson();
        p.eat();
        System.out.println("===============");
        SouthPerson s =(SouthPerson)p;
        s.eat();
        s.work();

        System.out.println("===============");
        p=new NorthPerson();
        p.eat();
        System.out.println("===============");
        NorthPerson n =(NorthPerson)p;
        n.eat();
        n.bath();
    }
}

看程序，写结果--1

class Fu {
    public void show() {
        System.out.println("fu show");
    }
}

class Zi extends Fu {
    public void show() {
        System.out.println("zi show");
    }

    public void method() {
        System.out.println("zi method");
    }
}

public class PolymorphicTest3 {
    public static void main(String[] args) {
        Fu f = new Zi();
//        f.method();//报错

        //向下转型
        Zi z = (Zi)f;
        z.method();//zi method
    }
}

看程序，写结果--2

class A1 {
    public void show() {
        show2();
    }

    public void show2() {
        System.out.println("我");
    }
}

class B1 extends A1 {

    //这里隐含了继承过来的show方法，只不过我们没有重写
//    public void show() {
//        show2();
//    }

    public void show2() {
        System.out.println("爱");
    }
}

class C1 extends B1 {
    public void show() {
        super.show();
    }

    public void show2() {
        System.out.println("你");
    }
}


public class PolymorphicTest4 {
    public static void main(String[] args) {
        A1 a = new B1();
        a.show(); // 爱（翻译看左，运行看右，找到A1中的show方法，运行B1中隐藏的继承自A1的show方法指向show2方法（就近原则））
        B1 b = new C1();
        b.show(); // 你（翻译看左，运行看右，找到B1中的show方法，运行C1中的show方法通过super关键字指向B1中的show2方法其实就是C1中继承自B1的show2方法，运行C1中的show2方法（就近原则））
    }
}