JAVA笔记8-对象转型casting

1、规则

(1)基类(父类)的引用类型变量可以指向其子类对象;(向上转型)

(2)基类的引用不可以访问其子类对象新增加的成员(属性和方法);

(3)可以使用“引用变量 instanceof 类名”来判断该引用变量所指向的对象是否属于该类(或该类的子类);

(4)子类对象可以当做基类对象来使用,即基类引用指向子类对象,是向上转型(upcasting);反之,向下转型(downcasting)。

例1:

public class TestAnimal{
    public static void main(String args[]){
        Animal a=new Animal("name");
        Dog d=new Dog("dogname","black");
        Cat c=new Cat("catname","blue");
        System.out.println(a instanceof Animal);//true
        System.out.println(d instanceof Animal);//true
        System.out.println(c instanceof Cat);//true
        System.out.println(a instanceof Cat);//false
        a=new Dog("bigyellow","yellow");//父类引用指向子类对象(向上转型)
        System.out.println(a.name);//bigyellow
        //System.out.println(a.furColor);//编译错误:找不到符号!规则(2)
        System.out.println(a instanceof Animal);//true
        System.out.println(a instanceof Dog);//true!!!
        Dog d1=(Dog) a;//强制转换符(向下转型)
        System.out.println(d1.furColor);//yellow
        //Cat c1=(Cat) a;//运行时抛异常!java.lang.ClassCastException:Dog cannot be cast to Cat
    }
}

class Animal{
    public String name;
    Animal(String name){
        this.name=name;
    }
}

class Dog extends Animal{
    public String furColor;
    Dog(String name,String furColor){
        super(name);
        this.furColor=furColor;
    }
}

class Cat extends Animal{
    public String eyesColor;
    Cat(String name,String eyesColor){
        super(name);
        this.eyesColor=eyesColor;
    }
}

 例2:对象转型带来的可扩展性方面的好处

public class TestAnimal{
    public static void main(String args[]){
        TestAnimal test=new TestAnimal();
        Animal a=new Animal("name");
        Dog d=new Dog("dogname","black");
        Cat c=new Cat("catname","blue");
        test.f(a);
        test.f(c);
        test.f(d);
    }
    public void f(Animal a){
        System.out.println("name:"+a.name);
        if(a instanceof Cat){
            Cat c1=(Cat) a;
            System.out.println(" "+c1.eyesColor+" eyes");
        }
        else if(a instanceof Dog){
            Dog d1=(Dog) a;
            System.out.println(" "+d1.furColor+" fur");
        }
    }
}

class Animal{
    public String name;
    Animal(String name){
        this.name=name;
    }
}

class Dog extends Animal{
    public String furColor;
    Dog(String name,String furColor){
        super(name);
        this.furColor=furColor;
    }
}

class Cat extends Animal{
    public String eyesColor;
    Cat(String name,String eyesColor){
        super(name);
        this.eyesColor=eyesColor;
    }
}

 

转载于:https://www.cnblogs.com/seven7seven/p/3656392.html

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