java随机取list中不重复的元素

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.huadoo.model.Area;
public class Aaaa {
 /**
  * @param args
  */
 public static void main(String[] args) {
  // TODO Auto-generated method stub
        List list=new ArrayList();
        Map map=new HashMap();
      
        Area area=new Area();
        area.setId(Short.parseShort("1"));
        area.setName("aaaa");
       
        Area area1=new Area();
        area1.setId(Short.parseShort("2"));
        area1.setName("bbb");
       
        Area area2=new Area();
        area2.setId(Short.parseShort("3"));
        area2.setName("ccc");
       
        list.add(area);
        list.add(area1);
        list.add(area2);
       
        int num=3;
        for(int a=0;a<num;a++)
        {
         int intnum=(int)(Math.random()*list.size());
            if(map.size()!=0)
            {
             for(int i=0;i<map.size();i++)