第2课第4节_Java面向对象编程_多态性_P【学习笔记】

摘要:韦东山android视频学习笔记 

面向对象程序的三大特性之继承性：

1、向上转换:只能定义被子类覆写的方法，不能调用在子类中定义的方法。

 class Father {
     private int money;    

     public int getMoney() {return money; }
     public void setMoney(int money) {this.money = money; }

     public void printInfo() {System.out.println("This is Father");}
 }

 class Son extends Father{
     public void printInfo() {System.out.println("This is son");}
     public void playGame(){System.out.println("This is son");}
 }

 class Daughter extends Father{
     public void printInfo() {System.out.println("This is Father");}
 }

 public class Cnv {
     public static void main (String args[]) {
         Son son = new Son();
         Daughter daughter = new Daughter();

         Father f = son;
         f.printInfo();
         //f.playGame();

         f = daughter;
         f.printInfo();
     }
 }

编译运行:

2、JAVA向下转换的例子,在进行对象的向下转换前，必须首先发生对象的向上转换.否则会编译不过.

 class Father {
     private int money;    

     public int getMoney() {return money; }
     public void setMoney(int money) {this.money = money; }

     public void printInfo() {System.out.println("This is Father");}
 }

 class Son extends Father{
     public void printInfo() {System.out.println("This is son");}
     public void playGame(){System.out.println("This is son");}
 }

 class Daughter extends Father{
     public void printInfo() {System.out.println("This is Father");}
 }

 public class Cnv2 {
     public static void main (String args[]) {
         Father f = new Son();
         Son son = (Son)f;
     }
 }

3、看一下，下面的例子，假如有一千个类继承了father这个类，如果我们要打印他们的信息，那样我们岂不是要写1千个print函数,下面的第二个代码则通过向上转换这个技巧实现.

 class Father {
     private int money;    

     public int getMoney() {return money; }
     public void setMoney(int money) {this.money = money; }

     public void printInfo() {System.out.println("This is Father");}
 }

 class Son extends Father{
     public void printInfo() {System.out.println("This is son");}
     public void playGame(){System.out.println("This is son");}
 }

 class Daughter extends Father{
     public void printInfo() {System.out.println("This is Dauhter");}
 }

 public class Cnv3 {
     public static void main (String args[]) {
         Father f = new Father();
         Son s = new Son();
         Daughter d = new Daughter();

         print(f);
         print(s);
         print(d);
     }

     public static void print(Father f){
         f.printInfo();
     }

     public static void print(Son s){
             s.printInfo();
     }

     public static void print(Daughter d){
             d.printInfo();
     }

 }

通过向上转换实现:

 class Father {
     private int money;    

     public int getMoney() {return money; }
     public void setMoney(int money) {this.money = money; }

     public void printInfo() {System.out.println("This is Father");}
 }

 class Son extends Father{
     public void printInfo() {System.out.println("This is son");}
     public void playGame(){System.out.println("This is son");}
 }

 class Daughter extends Father{
     public void printInfo() {System.out.println("This is Dauhter");}
 }

 public class Cnv4 {
     public static void main (String args[]) {
         Father f = new Father();
         Son s = new Son();
         Daughter d = new Daughter();

         print(f);
         print(s);
         print(d);
     }

     public static void print(Father f){
         f.printInfo();
     }
 }

上述两份代码编译运行结果一样:

4、instanceof: 用来判断一个对象是不是某个类的实例

 class Father {
     private int money;    

     public int getMoney() {return money; }
     public void setMoney(int money) {this.money = money; }

     public void printInfo() {System.out.println("This is Father");}
     public void drink(){System.out.println("drink");}
 }

 class Son extends Father{
     public void printInfo() {System.out.println("This is son");}
     public void playGame(){System.out.println("playGame");}
 }

 class Daughter extends Father{
     public void printInfo() {System.out.println("This is Dauhter");}
     public void dance(){System.out.println("dance");}
 }

 public class Cnv5{
     public static void main (String args[]) {
         Father f = new Father();
         Son s = new Son();
         Daughter d = new Daughter();

         printAction(f);
         printAction(s);
         printAction(d);
     }

     public static void printAction(Father f){
         if (f instanceof Son){
             Son s = (Son)f;
             s.playGame();
         }else if (f instanceof Daughter){
             Daughter d = (Daughter)f;
             d.dance();
         }else if(f instanceof Father){
             f.drink();
         }
     }
 }

编译运行结果:

相关代码存放在github，可以下载https://github.com/zzb2760715357/100ask