基础17instanceof用法和类的转换

instanceof用法和类的转换

instanceof作用：判断一个引用类型变量所指向的对象是否是一个类

四个代码一起食用

package com.oop.demo07;

public class Application {
          
   
    public static void main(String[] args) {
          
   
        //Object > String
        //Object > Person > Teacher
        //Object > Person > Student
        Object obj = new Student();

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

        Person per = new Student();
        System.out.println(per instanceof Student);//true
        System.out.println(per instanceof Person);//true
        System.out.println(per instanceof Object);//true
        System.out.println(per instanceof Teacher);//false
        //System.out.println(per instanceof String);//不可编译 Person 与String 没有血缘关系
        //编译成功与否根据左侧的类型，看他们是不是有血缘关系。右侧new的类型决定true和false
        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);
        //System.out.println(student instanceof String);

        Person person = new Person();
        System.out.println(person instanceof Student);//false
        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);
        System.out.println("======================================");

        /*
        1， 父类引用指向子类的对象
        2，把子类转换成父类，向上转型
        3，把父类转换成子类，向下转型：强制转换
        4，方便方法的调用，减少重复的代码！简洁

        核心思想：抽象
        封装、继承、多态      抽象类   接口
        */

        //类型之间的转换：父     子

        //高          低
        Person per1 = new Student();
        //Student 将这个对象转换为Student类型，我们就可以用Student类型里面的方法了！
        ((Student) per1).go();
        //子类转换成父类，可能丢失自己的本来的一些方法
        Student student1 = new Student();
        student1.go();
        Person per2 = student1;
        //per.go();方法丢失
    }
}

接下

package com.oop.demo07;

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

接下

package com.oop.demo07;

public class Student extends Person {
          
   
    public void go(){
          
   
        System.out.println("go");
    }
}

接下

package com.oop.demo07;

public class Teacher extends Person {
          
   
}