12.instanceof和类型转换

Posted 2021-03-14 duanfu

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

1 package com.oop.demo07;
2 
3 public class Person {
4 
5     public void run(){
6         System.out.println("run");
7     }
8 
9 }

1 package com.oop.demo07;
2 
3 public class Student extends Person {
4 
5     public void go() {
6         System.out.println("go");
7     }
8 
9 }

1 package com.oop.demo07;
2 
3 public class Teacher extends Person{
4 }

 1 package com.oop;
 2 
 3 import com.oop.demo07.Person;
 4 import com.oop.demo07.Student;
 5 import com.oop.demo07.Teacher;
 6 public class Application {
 7 
 8     public static void main(String[] args) {
 9 
10         //Object > String
11         //Object > Person > Teacher
12         //Object > Person > Student
13         Object object = new Student();
14 
15         //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！
16         System.out.println(object instanceof Student);//true
17         System.out.println(object instanceof Person);//true
18         System.out.println(object instanceof Object);//true
19         System.out.println(object instanceof Teacher);//False
20         System.out.println(object instanceof String);//False
21         System.out.println("================================");
22         Person person = new Student();
23         System.out.println(person instanceof Student);//true
24         System.out.println(person instanceof Person);//true
25         System.out.println(person instanceof Object);//true
26         System.out.println(person instanceof Teacher);//False
27         //System.out.println(person instanceof String);//编译报错
28         System.out.println("================================");
29         Student student = new Student();
30         System.out.println(student instanceof Student);//true
31         System.out.println(student instanceof Person);//true
32         System.out.println(student instanceof Object);//true
33         //System.out.println(student instanceof Teacher);//编译报错
34         //System.out.println(student instanceof String);//编译报错
35 
36     }
37 }

类型转换：

 1 package com.oop;
 2 
 3 import com.oop.demo07.Person;
 4 import com.oop.demo07.Student;
 5 
 6 public class Application {
 7 
 8     public static void main(String[] args) {
 9         //类型之间的转换：基本类型转换   父 子
10 
11         //高                    低
12         Person obj = new Student();
13 
14         //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！
15         //Student student = (Student) obj;
16         //student.go();
17         ((Student) obj).go();
18 
19 
20         //子类转换成父类会丢失一些独有的方法！
21         Student student = new Student();
22         student.go();
23         Person person = student;
24 
25     }
26 }

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！