java中的instanceof 的用法

1.instanceof功能：判断前面的对象是否属于后面的类，或者属于其子类；如果是，返回 true ，不是返回 false；

2.instanceof概念是在多态中提出的

3.注意事项：

使用 instanceof 时需要保证：
instanceof 前面的引用变量编译时的类型要么与后面的类型相同，要么与后面的类型具有父子继承关系，否则就会出现编译出错。

4,代码案例：

java 复制代码

//Student类
package demo07;

public class Student extends Person{
    public void go(){
    }
}

//Person类
package demo07;

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

//输出
import demo07.Person;
import demo07.Teacher;
import demo07.Student;

public class Application {
    public static void main(String[] args) {
        //System.out.println(X instanceof Y );能不能编译通过！
        //object>String
        //object>Person>Teacher
        //object>Person>Student
        Object object =new Student();
        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 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(student instanceof String);//编译报错
        System.out.println("==================");
        //类型之间的转换 父     子
        Person student1 = new Student();
        //student1.go();
        //将student这个对象转换成Student这个类型，然后就可以使用Student类型的方法了
        Student student11 = (Student) student1;//快捷键，(Student,要转换的类型)student,需要转换的对象    然后Alt+回车
        student11.go();
        Student student2 = new Student();
        //子类转换为父类，可能会丢失一些自己本来的方法
        student2.go();
        Person person1 =student2;
        //person1.go();
    }
}