Java instanof关键字详解

Java instanof关键字详解

instanof和类型转换

• instanof主要用于判断引用类型之间的关系
• （类型转换）引用类型，判断一个对象是什么类型

package oop.demo03;
//父类
public class Student {
}

package oop.demo03;
//子类High
public class High extends Student {
}

package oop.demo03;
//子类Junior
public class Junior extends Student {
}

package oop;
import oop.demo03.High;
import oop.demo03.Junior;
import oop.demo03.Student;

public class Application {
    public static void main(String[] args) {
        //Object类是所有类的父类
        //父类---子类---子子类
        //Object--Student--High
        //Object--Student--Junior
        //Object--String
        //父类引用指向子类对象
        Object object =new High();
        System.out.println(object instanceof High);//true
        System.out.println(object instanceof Student);//true
        System.out.println(object instanceof Object);//true
        System.out.println(object instanceof Junior);//false
        System.out.println(object instanceof String);//false

        System.out.println("==============");
        Student student= new High();
        System.out.println(student instanceof High);//true
        System.out.println(student instanceof Student);//true
        System.out.println(student instanceof Object);//true
        System.out.println(student instanceof Junior);//false
        //System.out.println(student instanceof String);//编译直接报错
    }
}

• 引用类型之间的转化

package oop.demo03;
//父类
public class Student {
}

package oop.demo03;
//子类High
public class High extends Student {
    //创建一个方法
    public int sum (int a,int b){
        return a+b;
    }
}

package oop;
import oop.demo03.High;
import oop.demo03.Student;

public class Application {
    public static void main(String[] args) {
        //Object--Student--High
        //Object--Student--Junior
        //Object--String
        //高（父）---------------低（子），低的可以直接调用高的方法
        Student student=new High();
        //s1为Student类型
        Student s1=new High();
        //s1.sum();//不能直接调用High类的方法
        //将s1强转为High类型，则可以直接调用High类的方法
        High high=(High) s1;
        high.sum(1,2);
        System.out.println(  high.sum(1,2));
    }
}

• 类型转换的总结
  • 父类引用指向子类的对象，需要引用子类的方法时，需要强转换
  • 把子类转换为父类，向上转换
  • 把父类转换为子类，向下转换，强制转换