排序有问题

public void testSort6(){

List<Student> studentList=new ArrayList<Student>();

Random random=new Random();

Integer id1=random.nextInt(100);

Integer id2;

Integer id3;

do{

id2=random.nextInt(100);

}while(id1==id2);

do{

id3=random.nextInt(100);

}while(id3==id2||id1==id3);

Student st1=new Student(id1+"","Jack");

Student st2=new Student(id2+"","Lucy");

Student st3=new Student(id3+"","Silence");

studentList.add(st1);

studentList.add(st2);

studentList.add(st3);

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

for (Student student : studentList) {

System.out.println("学生为"+student.id+"  "+student.name);

}

Collections.sort(studentList);

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

for (Student student2 : studentList) {

System.out.println("学生为"+student2.id+"  "+student2.name);

}

//运行了好几次结果有时会出错，哪里有问题吗

//某次出错结果如下

--------排序前--------

学生为24  Jack

学生为18  Lucy

学生为4  Silence

--------排序后--------

学生为18  Lucy

学生为24  Jack

学生为4  Silence