送分啊简单java 有关问题

送分啊....简单java 问题
package   com.test;
import   java.util.*;
public   class   test   {
public   static   void   main   (String[]   args){
ArrayList   arraylist=new   ArrayList();
for   (int   i   =   0;   i   <10   ;i++)   {
  Article   article=new   Article(i+ " ", "article "+i);
  Article   article1=new   Article(i+ " ", "article "+i);
arraylist.add(article);
arraylist.add(article1);
}
Iterator   iterator=arraylist.iterator();

ArrayList   a1=null;
int   i=0;
while(iterator.hasNext()){
Article   article=(Article)iterator.next();
String   s1=article.getId();
System.out.println(s1);
a1.add(s1);   //这里为什么会错
}
}
}

------解决方案--------------------
a1没有初始化
------解决方案--------------------
a1是个空对象啊,会报NullPointException
------解决方案--------------------
null.add(s1)?


ArrayList a1 = new ArrayList();