一.Android Java基础_多态性

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

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Cnv.java 
root@ubuntu:/home/topeet/guyilian# java Cnv
This is son
This is daughter

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("playGame ...");}}class Daughter extends Father{public void printInfo() {System.out.println("This is daughter");}}public class Cnv2 {public static void main (String args[]) {Father f = new Son();Son son = (Son)f;   /* Son son = new Son(); */}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Cnv2.java 
root@ubuntu:/home/topeet/guyilian# java Cnv2

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

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Cnv3.java 
root@ubuntu:/home/topeet/guyilian# java Cnv3
This is Father
This is son
This is daughter

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

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Cnv4.java 
root@ubuntu:/home/topeet/guyilian# java Cnv4
This is Father
This is son
This is daughter

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 daughter");}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 son = (Son)f;son.playGame();}else if (f instanceof Daughter) {Daughter d = (Daughter)f;d.dance();}else if (f instanceof Father) {f.drink();}}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Cnv5.java 
root@ubuntu:/home/topeet/guyilian# java Cnv5
drink ...
playGame ...
dance ...