复习

其中登记式注册单例模式：

import java.util.*;

class Single{

       private static HashMap hashmap = new HashMap();

       private int number = 0;

       static{

              Single single = new Single();

              hashmap.put(single.getClass().getName(),single);

       }

       private Single(){}

       public static Single getInstance(String name){

              if(name == null){

                     name = "Single";

              }

              if(hashmap.get(name) == null){

                     try{

                            hashmap.put(name,Class.forName(name).newInstance());

                     }catch(Exception e){

                            System.out.println(e.getMessage());

                     }

              }

              return (Single)hashmap.get(name);

       }

       public void fun(){

              number++;

              System.out.println("the is the single fun :" + number);

       }

}

他的另外一个做法是同过继承，通过父类来注册子类的对象：

代码如：

class SingleChild extends Single{

       public SingleChild(){}

       public static SingleChild getInstance(){

              return (SingleChild)Single.getInstance("SingleChild");

       }

       public void fun(){

              System.out.println("singlechild");

       }

}

当它要这样做之后就破坏了父类，因为子类要调用父类的构造子函数，所以父类的构造子函数就不能够为私有的，这就破坏单例模式的优点。