下面是singleton pattern的原意
package pattern.creational.singleton.demo1;
/**
* <p>title: the singleton pattern</p>
*
* <p>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.
*
* </p>
*
* <p>copyright: copyright (c) 2005</p>
*
* <p>company: </p>
*
* @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;
/**
* <p>title: singleton pattern 的变形</p>
*
* <p>description: </p>
*
* <p>copyright: copyright (c) 2005</p>
*
* <p>company: </p>
*
* @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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