java實現Comparable介面問題

package com.rae;import java.util.ArrayList;import java.util.Collection;import java.util.Collections;import java.util.List;import java.util.TreeSet;import java.util.Iterator;public class Student implements Comparable<Student> {        private int id;        private String name;        public Student (int id, String name) {                this.id = id;                this.name = name;        }        public void setId (int id) {                this.id = id;        }        public void setName (String name) {                this.name = name;        }        public int getId () {                return id;        }        public String getName () {                return name;        }        public String toString () {                return (id + "\t" + name);        }        public int compareTo (Student arg) {                return id - arg.id;        }               public static void main (String args[]) {                TreeSet<Student> tset = new TreeSet<Student>();                tset.add(new Student(5, "Tom"));                tset.add(new Student(3, "John"));                tset.add(new Student(9, "David"));                tset.add(new Student(7, "Alice"));                List<Student> list = new ArrayList<Student>();                list.add(new Student(5, "Tom"));                list.add(new Student(3, "John"));                list.add(new Student(9, "David"));                list.add(new Student(7, "Alice"));                Iterator<Student> itor = tset.iterator();                while (itor.hasNext()) {                        System.out.println(itor.next().toString());                }                Collections.sort(list);                itor = list.iterator();                while(itor.hasNext()){                System.out.println(itor.next().toString());                }        }}