为什么我每次运行都是2、4、1这个顺序，不是应该变吗？

代码如下：

package com.imooc_collection;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.Scanner;

public class SetTest {

public List<Course> coursesToSelect;

public SetTest() {

coursesToSelect = new ArrayList<Course>();

}

/**

* 用于往coursesToSelect中添加备选课程

*/

public void testAdd() {

// 创建一个课程对象，并通过调用add方法，添加到备选课程List中

Course cr1 = new Course("1", "数据结构");

coursesToSelect.add(cr1);

Course temp = (Course) coursesToSelect.get(0);

// System.out.println("添加了课程：" + temp.id + ":" + temp.name);

Course cr2 = new Course("2", "C语言");

coursesToSelect.add(0, cr2);

Course temp2 = (Course) coursesToSelect.get(0);

// System.out.println("添加了课程：" + temp2.id + ":" + temp2.name);

// coursesToSelect.add(cr1);

// Course temp0 = (Course) coursesToSelect.get(2);

// System.out.println("添加了课程：" + temp.id + ":" + temp.name);

// 以下方法会抛出数组下标越界异常

// Course cr3 = new Course("3", "test");

// coursesToSelect.add(4, cr3);

Course[] course = { new Course("3", "离散数学"), new Course("4", "汇编语言") };

coursesToSelect.addAll(Arrays.asList(course));

Course temp3 = (Course) coursesToSelect.get(2);

Course temp4 = (Course) coursesToSelect.get(3);

// System.out.println("添加了两门课程：" + temp3.id + ":" +

// temp3.name + ";" + temp4.id + ":" + temp4.name);

Course[] course2 = { new Course("5", "高等数学"), new Course("6", "大学英语") };

coursesToSelect.addAll(2, Arrays.asList(course2));

Course temp5 = (Course) coursesToSelect.get(2);

Course temp6 = (Course) coursesToSelect.get(3);

// System.out.println("添加了两门课程：" + temp5.id + ":" +

// temp5.name + ";" + temp6.id + ":" + temp6.name);

}

/**

* 通过for each方法访问集合元素

* 

* @param args

*/

public void testForEach() {

System.out.println("有如下课程待选(通过for each访问)：");

for (Object obj : coursesToSelect) {

Course cr = (Course) obj;

System.out.println("课程：" + cr.id + ":" + cr.name);

}

}

/**

* @param args

*/

public static void main(String[] args) {

SetTest st = new SetTest();

st.testAdd();

st.testForEach();

// 创建一个学生对象

Student student = new Student("1", "小明");

System.out.println("欢迎学生：" + student.name + "选课！");

// 创建一个Scanner对象，用来接收从键盘输入的课程ID

Scanner console = new Scanner(System.in);

for (int i = 0; i < 3; i++) {

System.out.println("请输入课程ID");

String courseId = console.next();

for (Course cr : st.coursesToSelect) {

if (cr.id.equals(courseId)) {

student.courses.add(cr);

/**

* Set中，添加某个对象，无论添加多少次， 最终只会保留一个该对象（的引用）， 并且，保留的是第一次添加的那一个

*/

// student.courses.add(null);

student.courses.add(cr);

}

}

}

st.testForEachForSet(student);

}

public void testForEachForSet(Student student) {

// 打印输出，学生所选的课程！

//System.out.println("共选择了：" + student.courses.size() + "门课程！");

for (Course cr : student.courses) {

System.out.println("选择了课程：" + cr.id + ":" + cr.name);

}

}

}