instanceof

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

public class Student extends Person{
}

public class Teacher extends Person{
}

public class Application {
    public static void main(String[] args) {
        // instanceof 是 Java 的保留关键字。它的作用是测试它左边的对象是否是它右边的类的实例，返回 boolean 的数据类型。

        // Object > String
        // Object > Person > Teacher
        // Object > Person > Student
        Object object = new Student();

        System.out.println(object instanceof Student); // true
        System.out.println(object instanceof Person); // true
        System.out.println(object instanceof Object); // true
        System.out.println(object instanceof Teacher); // false
        System.out.println(object instanceof String); // false

        System.out.println("====================================");
        Person person = new Student();
        System.out.println(person instanceof Student); // true
        System.out.println(person instanceof Person); // true
        System.out.println(person instanceof Object); // true
        System.out.println(person instanceof Teacher); // false
//        System.out.println(person instanceof String); // 编译报错

        System.out.println("====================================");
        Student student = new Student();
        System.out.println(student instanceof Student); // true
        System.out.println(student instanceof Person); // true
        System.out.println(student instanceof Object); // true
//        System.out.println(student instanceof Teacher); // 编译报错
//        System.out.println(student instanceof String); // 编译报错

    }

}

类型转换

public class Person {

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

public class Student extends Person{

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

public class Teacher extends Person{
}

public class Application {
    public static void main(String[] args) {
        // 类型之间的转换  父 > 子

        // Person 高           Student 低
        Person obj = new Student();
//        obj.go(); //报错
        // student将这个对象转换为Student类型，我们就可以使用Student类型的方法了
//        Student student = (Student)obj;
//        student.go();
        // 简写
        ((Student) obj).go();


        // 子类转换为父类，可能丢失自己的本来的一些方法
        // 低转高，直接转换
        Student student1 = new Student();
        student1.go();
        Person person = student1;
    }

}

多态
	1. 父类的引用指向子类的对象（子类的引用不能指向父类的对象）
	2. 把子类转换为父类，向上转型
	3. 把父类转换为子类，向下转换；强制转换
	4. 方便方法的调用，减少重复的代码，简洁

https://www.bilibili.com/video/BV12J41137hu?p=72