枚举实现单例简单例子

import java.util.Date;


public enum Datetest{
	INSTANCE;
	private Date d;
	private  Datetest(){
	 d=new Date();
        System.out.println("调用了构造方法");
		
	}
	public Date builddate(){  
        
        return d;  
    }  
};

 然后定义一个类来调用它

 import java.util.*;

public class test{
	public static void main(String args[]){
	
	Date d=Datetest.INSTANCE.builddate();
	Date d1=Datetest.INSTANCE.builddate();
	System.out.println(d.toString()+"  "+d1.toString());
	d1.setHours(11);
	System.out.println(d==d1);
	System.out.println(d.toString()+"  "+d1.toString());
	}
}

 结果

调用了构造方法
Mon Jun 29 10:47:45 GMT+08:00 2015  Mon Jun 29 10:47:45 GMT+08:00 2015
true
Mon Jun 29 11:47:45 GMT+08:00 2015  Mon Jun 29 11:47:45 GMT+08:00 2015