单例模式学习及控制实例化对象个数
单例模式学习：

package com.amaker.single;import java.util.HashMap;import java.util.Map;public class People {//===============================================================//标准的单例private static People p = null;private People(){}public static People getInstance(){if(p==null){p = new People();}return p;}//===============================================================//利用缓存的单例private static Map<String,People> map = new HashMap<String,People>();public static People getInstance2(){People p = map.get("one");if(p==null){p = new People();map.put("one", p);}return p;}//===============================================================//实例化三个或更多实例，可以按要求实例化对象个数,如下面的count可以从配置文件中读取private static int number = 1;private static int count = 3;public static People getInstance3(){p = map.get(number+"");if(p==null){p = new People();map.put(number+"", p);}number++;if(number>count){number = 1;}return p;}}

package com.amaker.client;import com.amaker.single.People;public class Test {public static void main(String[] args) {//单例测试：/*People p1 = People.getInstance();System.out.println("p=="+p1);People p2 = People.getInstance();System.out.println("p=="+p2);People p3 = People.getInstance();System.out.println("p=="+p3);*///缓存测试：/*People p1 = People.getInstance2();System.out.println("p=="+p1);People p2 = People.getInstance2();System.out.println("p=="+p2);People p3 = People.getInstance2();System.out.println("p=="+p3);*///控制对象的个数,暂时实例化三个对象People p1 = People.getInstance3();System.out.println("p=="+p1);People p2 = People.getInstance3();System.out.println("p=="+p2);People p3 = People.getInstance3();System.out.println("p=="+p3);People p4 = People.getInstance3();System.out.println("p=="+p4);People p5 = People.getInstance3();System.out.println("p=="+p5);}}