1. 父类引用指向子类对象

2. 把子类转换成父类，向上转型

3. 吧父类转换成子类，向下转型：强制转换

4. 方便方法的调用

封装 继承 多态

import Dome.Person;
import Dome.Student;
import Dome.Teacher;
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public class Application {
    public static void main(String[] args) {
       Person obj = new Student();
        ((Student)obj).go();
    }
}

package Dome;
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public class Teacher extends Person{
}
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/*
 public static void main(String[] args) {
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            Object object = new Object();
            System.out.println(object instanceof Student);
            System.out.println(object instanceof Person);
            System.out.println(object instanceof Object);
            System.out.println(object instanceof Teacher);
            System.out.println(object instanceof String);
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 */

instanceof 和类型转换