模拟hibernate 往集合里加元素???在读集合里的,但是确抛除java.lang.NullPo

我试着写了个类 只是定义了个id 和本生类的集合, 写了个 生成集合的 在从里面读出来,但是确出错了
    Exception in thread "main" java.lang.NullPointerException
at abin.bbs.database.test.myforumTest.printForum2tree(myforumTest.java:57)
at abin.bbs.database.test.myforumTest.printForum2tree(myforumTest.java:58)
at abin.bbs.database.est.myforumTest.main(myforumTest.java:26)
我源码贴出来 请问怎么改进
import java.lang.Comparable;
import java.io.Serializable;
import java.util.Set;
public class Forum2 implements Comparable, Serializable {
    private Integer  id;
    private Set forum;
    private int hashCode = Integer.MIN_VALUE;

public Forum2 () {
initialize();
}
protected void initialize () {}

public Integer getId() {
return id;
}

public void setId(Integer id) {
this.id = id;
}

public Set getForum() {
return forum;
}

public void setForum(Set forum) {
this.forum = forum;
}
public void addToForums(Forum2 o){
       if(this.forum==null) this.forum= new java.util.HashSet();
getForum().add(o);
}
public boolean equals (Object obj) {
if (null == obj) return false;
if (!(obj instanceof abin.bbs.database.test.Forum2)) return false;
else {
   abin.bbs.database.test.Forum2 forum = (abin.bbs.database.test.Forum2) obj;
   if (null == this.getId() || null == forum.getId()) return false;
   else return (this.getId().equals(forum.getId()));
    }
}

public int hashCode () {
  if (Integer.MIN_VALUE == this.hashCode) {
     if (null == this.getId()) return super.hashCode();
     else {
String hashStr = this.getClass().getName() + ":" + this.getId().hashCode();
this.hashCode = hashStr.hashCode();
}
     }
return this.hashCode;
}

public int compareTo (Object obj) {
if (obj.hashCode() > hashCode()) return 1;
else if (obj.hashCode() < hashCode()) return -1;
else return 0;
}

public String toString () {
return super.toString();
}


}
import java.util.Collection;
import java.util.Date;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.HashSet;


import abin.bbs.database.test.Forum2;



[align=left]public class myforumTest {
 
  // private Forum2 Forum2;
   private Forum2 root;
   public myforumTest(){}
      

   public static void main(String[] args) {
   myforumTest t=new myforumTest();
   //System.out.println(t.addtree().getForum().size());
    t.printForum2tree(t.addtree().getForum());
   //t.testtree();

}
   public Forum2 addtree(){
   root=new Forum2();
   root.setId(new Integer(0));
   Forum2 child=new Forum2();
   child.setId(new Integer(1));
   //root.setForum(null);
   root.addToForums(child);
 
for(int i=0;i<3;i++){
Forum2 childson=new Forum2();
  childson.setId(new Integer(i+2));  
child.addToForums(childson);
    
}

return root;

   }
      public void printForum2tree(Set node){
     
      Iterator it=node.iterator();
   
while(it.hasNext()){
Forum2 fe= (Forum2)it.next();
System.out.print(fe.getId());
Set children=fe.getForum();

if(children.size()>0){
printForum2tree(children);
    System.out.print(" ");
   }
}
      }
  
}[align]

请留下各位的影子 好嘛?

最好格式一下代码，看的比较累，有点没看明白，
好像是private Set forum;
public void setForum(Set forum) {
this.forum = forum;
}
但是好像没有看到那里调用了
root=new Forum2();
root.setId(new Integer(0));
Forum2 child=new Forum2();
child.setId(new Integer(1));
//root.setForum(null);
root.addToForums(child);
被屏蔽le