Java(15) 多态

Posted 2020-09-23 祝阿明

一.多态

   1.1 多态：一个事物的多种形态

   1.2 多态的表现

        行为(方法)多态：重写和重载。

        引用多态(动态绑定):编译时的类型和运行时不一致的这种现象

       例：父类 = 子类

             Pet p = new Dog();

　　向上转型；父类=子类

 　　 例：Pet p = new Dog();

     向下转型：子类 = （子类）父类

        例: Dog d = (Dog)p;

 1 public abstract class Pet {
 2     private String name;
 3     private int health=100;
 4     private int age;
 5     private int love=50;
 6     public Pet(){
 7         
 8     }
 9     public Pet(String name,int health,int age,int love){
10         this.name = name;
11         this.health = health;
12         this.age = age;
13         this.love = love;
14     }
15     public String getName(){
16         return name;
17     }
18     public void setName(String name){
19         this.name= name;
20     }
21     public int getHealth(){
22         return health;
23     }
24     public void setHealth(int health){
25         this.health = health;
26     }
27     public int getAge(){
28         return age;
29     }
30     public void setAge(int age){
31         this.age = age;
32     }
33     public int getLove(){
34         return love;
35     }
36     public void setLove(int love){
37         this.love = love;
38     }
39     public void ji(){
40         System.out.println("宠物的自白：我的名字叫："+this.name+",年龄为："+this.age+"和主人的亲密度为："+this.love);
41     }
42     public abstract void eat();
43 }

宠物抽象类

 1 public class Dog extends Pet{
 2     private String strain;
 3     public Dog(){
 4         
 5     }
 6     public Dog(String name,int health,int age,int love,String strain){
 7         super(name,health,age,love);
 8         this.strain = strain;
 9     }
10     public void catchingFlyDisc(){
11         System.out.println("接飞盘");
12         int health = getHealth();
13         health = health-10;
14         setHealth(health);
15         
16         int love = getLove();
17         love = love+5;
18         setLove(love);
19     }
20     public void eat(){
21         System.out.println("狗狗吃鱼");
22     }
23     public String getStrain(){
24         return strain;
25     }
26     public void setStrain(String strain){
27         this.strain = strain;
28     }
29 
30 }

狗狗类继承宠物类

 1 public class Penguin extends Pet {
 2     private String sex;
 3     public Penguin(){
 4         
 5     }
 6     public Penguin(String name,int health,int age,int love,String sex){
 7         super(name,health,age,love);
 8         this.sex = sex;
 9     }
10     public void swimming(){
11         System.out.println("游泳");
12         int health = getHealth();
13         health = health-10;
14         setHealth(health);
15         
16         int love = getLove();
17         love = love+5;
18         setLove(love);
19     }
20     public void eat(){
21         System.out.println("企鹅喝水");
22     }
23     public String getSex(){
24         return sex;
25     }
26     public void setSex(String sex){
27         this.sex = sex;
28     }
29 
30 }

企鹅类继承宠物类

 1 public class Master {
 2     public void feed(Pet p){
 3         p.eat();
 4     }
 5     public void play(Pet p){
 6         if(p instanceof Dog ){
 7             Dog d = (Dog) p;
 8             d.catchingFlyDisc();
 9         }else if(p instanceof Penguin){
10             Penguin pe = (Penguin) p;
11             pe.swimming();
12         }
13     }
14 }

主人类

 1 public class TestPMPD {
 2     public static void main(String[] args){
 3         Master m= new Master();
 4         //向下转型
 5         Pet p = new Dog();
 6         Dog d = (Dog) p;
 7         m.feed(d);
 8 //        Pet p = new Penguin();
 9 //        m.feed(p);
10     }
11 }

测试类

运行结果为： 

  注意：java.lang.ClassCastException:类型转换异常

          父类的运行时类型跟要转成的子类的类型不一致

  注意：引用多态存在继承关系

二.instanceof

  语法： 对象A instanceof 类型B

      判断对象A的类型是否是类型B，是就返回true，否则返回false

例：d instanceof Dog

三.实现多态的2种形式

  a.使用父类作为方法形参实现多态

    public void play(Pet p){...}

  b.使用父类作为方法返回值实现多态

    public Pet getPet(int type){...}