TreeSet 为什么在使用subSet()时出现指针错误，程序如下

package LianXi2;

import java.util.Iterator;
import java.util.TreeSet;

public class UpdateStu implements Comparable {
    String name;
    long id;
   

   


    private UpdateStu(String name, long id) {
        super();
        this.name = name;
        this.id = id;
    }

    public int compareTo(Object o) {
        UpdateStu updat = (UpdateStu) o;
        int result = id > updat.id?1:(id>updat.id?0:-1);
        return result;
    }

   

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public long getId() {
        return id;
    }

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

    public static void main(String[] args) {
        UpdateStu stu1 = new UpdateStu("王小明",200);
        UpdateStu stu2 = new UpdateStu("王2明",500);
        UpdateStu stu3 = new UpdateStu("王3明",3400);
        UpdateStu stu4 = new UpdateStu("王4明",120);
        UpdateStu stu5 = new UpdateStu("王5明",530);
      

        TreeSet tree = new TreeSet();
        tree.add(stu1);
        tree.add(stu2);
        tree.add(stu3);
        tree.add(stu4);
        tree.add(stu5);
      

        Iterator it = tree.iterator();
        while(it.hasNext()){
            UpdateStu stu = (UpdateStu) it.next();
            System.out.println(stu.getName()+""+stu.getId());
        }
      

        System.out.println("---------------------------------------");
      

        it = tree.headSet(stu2).iterator();
        while(it.hasNext()){
            UpdateStu stu = (UpdateStu) it.next();
            System.out.println(stu.getName()+""+stu.getId());
        }
      

        System.out.println("---------------------------------------");
      

        it = tree.subSet(stu2,stu3).iterator();
        while(it.hasNext()){
            UpdateStu stu = (UpdateStu) it.next();
            System.out.println(stu.getName()+""+stu.getId());
        }

    }

}