生成3个不重复的1000以内的随机正整数作为学生ID的程序，请多多指教！！！

public void sort4(){
  List<Student> studentList=new ArrayList<Student>();
  String s="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXY";
  Random random=new Random();
  Integer k;
  for(int i=0;i<3;i++){
    StringBuffer str=new StringBuffer();
   do{
    k=random.nextInt(1000);
    for(int j=0;j<4j++){
     int num=random.nextInt(s.length());
     str.append(s.charAt(num));  } 
    
   }while(studentList.contains(k)&&studentList.contains(str.toString()));
  
  
  studentList.add(new Student(k+"",str.toString()));
  }
  
  System.out.println("------------排序前-----------------");
  for(Student student:studentList){
   System.out.println("学生："+student.id+","+student.name);
  }
  Collections.sort(studentList);
  System.out.println("------------排序后-----------------");
  for(Student student:studentList){
   System.out.println("学生："+student.id+","+student.name);
  }
  Collections.sort(studentList,new StudentComparator());
  System.out.println("--------------按照姓名排序后----------------");
  for(Student student:studentList){
   System.out.println("学生："+student.id+","+student.name);
  }
  
  }