12.instanceof和类型转换

Instanceof：

判断一个对象是什么类型的~，可以判断两个类之间是否存在父子关系

package com.oop.demo07;

public class Person {

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

}

package com.oop.demo07;

public class Student extends Person {

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

}

package com.oop.demo07;

public class Teacher extends Person{
}

package com.oop;

import com.oop.demo07.Person;
import com.oop.demo07.Student;
import com.oop.demo07.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);//能不能编译通过！取决于X和Y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(student instanceof String);//编译报错

    }
}

类型转换：

package com.oop;

import com.oop.demo07.Person;
import com.oop.demo07.Student;

public class Application {

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

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

        //student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();
        ((Student) obj).go();


        //子类转换成父类会丢失一些独有的方法！
        Student student = new Student();
        student.go();
        Person person = student;

    }
}

多态小结：

1、前提：父类引用指向子类对象

2、把子类转换为父类，向上转型：会丢失一些独有的方法

3、把父类转换为子类，向下转型：强制转换

好处：

方便方法的调用，减少重复的代码，可以有效的提升利用率！是代码变的简洁

抽象：编程思想：三大特性，封装、继承、多态！ 后面有抽象类！接口！