浅读java编程思想之多态

8.1向上转型：父类对象通过子类对象去实例化，向上转型是不需要进行强制转换的，但是向上转型会丢失精度。

测试一：每个类的实例都经由ride（）方法向上转型为Bike

public class Test {
    public static void ride(Cycle c) {
    }

    public static void main(String[] args) {
        ride(new Cycle());
        ride(new Unicycle());
        ride(new Bicycle());
        ride(new Tricycle());
    }
}
class Cycle {


}
class Unicycle extends Cycle {

}

class Bicycle extends Cycle {
}

class Tricycle extends Cycle {
}

 

测试二：验证多态对方法的有作用

public class Test {
    public static void ride(Cycle c) {
        System.out.println("Num. of wheels: " + c.wheels());
    }

    public static void main(String[] args) {
        ride(new Cycle());
        ride(new Unicycle());
        ride(new Bicycle());
        ride(new Tricycle());
    }
}
class Cycle {
public int wheels(){
    return 0;
}

}
class Unicycle extends Cycle {
    public int wheels(){
        return 1;
}
}
class Bicycle extends Cycle {
    public int wheels(){
        return 2;
}
}

class Tricycle extends Cycle {
    public int wheels(){
        return 3;
}
}
输出：

Num. of wheels: 0
Num. of wheels: 1
Num. of wheels: 2
Num. of wheels: 3

测试三

class Person {
     public void talk(){
         System.out.println("我是人，在说话");
     }
}
 class Children extends Person{
     public void talk(){
         System.out.println("我是小孩，在说话");
     }
     public static void main(String[] args) {
        Person p=new Children();
        p.talk();
    }
 }

输出结果
我是小孩，在说话

 

向下转型 （父类的对象可以转换为子类的对象，但是必须进行强制类型转换）

 class Person {
     public void talk(){
         System.out.println("我是人，在说话");
     }
}
 class Children extends Person{
     public void talk(){
         System.out.println("我是小孩，在说话");
     }
     public void talk2(){
         System.out.println("我是第二个小孩，在说话");
     }
     public static void main(String[] args) {
        Person p=new Children();
        //将p对象向下转型
        Children c=(Children)p;
        p.talk();
    }
 }