instanceof和类型转换

instanceof

package com.oop;

import com.oop.demo06.Person;
import com.oop.demo06.Student;
import com.oop.demo06.Teacher;

public class Application {
    
public static void main(String[] args) {

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

        //System.out.println(X instanceof Y);//编译能不能通过，看XY之间是否存在父子关系


        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(person instanceof String);//编译报错！

    }
}    

类型转换

Application.java

package com.oop;

import com.oop.demo06.Person;
import com.oop.demo06.Student;

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

        //高                  低
        Person obj = new Student();

        //obj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();//go
        //((Student)obj).go();//两句写为一句


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

    }
}

/*
    1.父类引用指向子类的对象
    2.子类转换为父类，向上转型；直接转
    3.父类转换为子类，向下转型；强制转换  可能会丢失一些子类本来的方法
    4.方便方法的调用，减少重复的代码！简洁
 */

Student.java

package com.oop.demo06;

public class Student extends Person {

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

}

Person.java

package com.oop.demo06;

public class Person {

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