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！