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谈模式(Singleton Pattern)的变形


  下面是Singleton Pattern的原意
  package Pattern.Creational.Singleton.Demo1;
  
  /**
   *

Title: THE SINGLETON PATTERN


   *
   *

Description: 建议使用这个方法
   * Another approach, suggested by Design Patterns, is to create
   * Singletons using a static method to issue and keep track of instances. To
   * prevent instantiating the class more than once, we make the constructor
   * private so an instance can only be created from within the static method
   * of the class.
   *
   * Other Consequences of the Singleton Pattern
   * 1. It can be difficult to subclass a Singleton, since this can only work
   * if the base Singleton class has not yet been instantiated.
   * 2. You can easily change a Singleton to allow a small number of instances
   * where this is allowable and meaningful.
   *
   *


   *
   *

Copyright: Copyright (c) 2005


   *
   *

Company:


   *
   * @author Lin.Xiang.Xiang
   * @version 1.0
   */
  public class IsSpooler
  {
  //this is a prototype for a printer-spooler class
  //such that only one instance can ever exist
   static boolean instance_flag = false; //true if 1 instance
  //the constructor is privatized-
  //but need not have any content
   private IsSpooler() {}
  
  //static Instance method returns one instance or null
   static public IsSpooler Instance() {
    if (!instance_flag) {
     instance_flag = true;
     return new IsSpooler(); //only callable from within
    }
    else
     return null; //return no further instances
   }
  
  //-------------------------------------------
   public void finalize() {
    instance_flag = false;
   }
  
   public static void main(String[] args) {
   }
  }
  
  
  
  只要稍加修改,我们可控制只创建N个实例,N由我们来定.
  下面给出代码示范
  
   package Pattern.Creational.Singleton.Demo2;
  
  /**
   *

Title: Singleton Pattern 的变形


   *
   *

Description:


   *
   *

Copyright: Copyright (c) 2005


   *
   *

Company:


   *
   * @author Lin.Xiang.Xiang
   * @version 1.0
   */
  public class N_Instance {
   final static int MAXINSTANCE = 5; //最多只能创建5个实例
   static int instanceCount = 0; //开始无实例
  
   private N_Instance() {}
  
   static public N_Instance getInstance() {
    if (instanceCount < MAXINSTANCE) {
     instanceCount++;
     return new N_Instance(); //返回一个实例
    }
    else
     return null; //返回空
   }
  
   public void finalize() {
    instanceCount--;
   }
  }
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