1. 父類引用指向子類物件

2. 把子類轉換成父類，向上轉型

3. 吧父類轉換成子類，向下轉型：強制轉換

4. 方便方法的呼叫

封裝 繼承 多型

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

package Dome;
​
public class Teacher extends Person{
}
​
​
​
​
​
​
​
​
​
​
​
​
/*
 public static void main(String[] args) {
​
       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);
​
 */