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 > StudentObject object = new Student();System.out.println(object instanceof Student); // trueSystem.out.println(object instanceof Person); // trueSystem.out.println(object instanceof Object); // trueSystem.out.println(object instanceof Teacher); // falseSystem.out.println(object instanceof String); // falseSystem.out.println("====================================");Person person = new Student();System.out.println(person instanceof Student); // trueSystem.out.println(person instanceof Person); // trueSystem.out.println(person instanceof Object); // trueSystem.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); // trueSystem.out.println(student instanceof Person); // trueSystem.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