instanceof关键字和向上向下转型

public class Computer {//电脑

    public static void main(String[] args) {
        Pad pad = new Pad();
        HuaWeiPad huaWeiPad = new HuaWeiPad();

        System.out.println("pad是否继承自电脑？"+(pad instanceof Computer));
        System.out.println("pad是否继承自电脑？"+(huaWeiPad instanceof Pad));
        System.out.println("pad是否继承自电脑？"+(huaWeiPad instanceof Computer));
        System.out.println("pad是否继承自电脑？"+(pad instanceof HuaWeiPad));

    }

}

class Pad extends Computer{//平板电脑

}
class HuaWeiPad extends Pad{//华为平板电脑

}

public class Person {
    public Person(String name){
        System.out.println("你好，我叫"+name);
    }
}

public class Student extends Person {

    public Student(String name) {
        super(name);
    }
}

public class Doctor extends Person{

    public Doctor(String name) {
        super(name);
    }
}

public class Test {
    public static void main(String[] args) {
        Person tom = new Student("tom");//向上转型，由父类声明了一个对象，由子类来实例化
        Student st = (Student) tom;

       Person jack = new Doctor("jack");//向下转型，强制转型，父类对象要先引用这个子类对象
        Doctor dr = (Doctor) jack;
    }
}