為什么我每次運行都是2、4、1這個順序，不是應該變嗎？

代碼如下：

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中添加備選課程

*/

public void testAdd() {

// 創(chuàng)建一個課程對象，并通過調用add方法，添加到備選課程List中

Course cr1 = new Course("1", "數(shù)據(jù)結構");

coursesToSelect.add(cr1);

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

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

Course cr2 = new Course("2", "C語言");

coursesToSelect.add(0, cr2);

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

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

// coursesToSelect.add(cr1);

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

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

// 以下方法會拋出數(shù)組下標越界異常

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

// coursesToSelect.add(4, cr3);

Course[] course = { new Course("3", "離散數(shù)學"), new Course("4", "匯編語言") };

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

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

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

// System.out.println("添加了兩門課程：" + temp3.id + ":" +

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

Course[] course2 = { new Course("5", "高等數(shù)學"), new Course("6", "大學英語") };

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

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

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

// System.out.println("添加了兩門課程：" + temp5.id + ":" +

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

}

/**

* 通過for each方法訪問集合元素

*?

* @param args

*/

public void testForEach() {

System.out.println("有如下課程待選(通過for each訪問)：");

for (Object obj : coursesToSelect) {

Course cr = (Course) obj;

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

}

}

/**

* @param args

*/

public static void main(String[] args) {

SetTest st = new SetTest();

st.testAdd();

st.testForEach();

// 創(chuàng)建一個學生對象

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

System.out.println("歡迎學生：" + student.name + "選課！");

// 創(chuàng)建一個Scanner對象，用來接收從鍵盤輸入的課程ID

Scanner console = new Scanner(System.in);

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

System.out.println("請輸入課程ID");

String courseId = console.next();

for (Course cr : st.coursesToSelect) {

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

student.courses.add(cr);

/**

* Set中，添加某個對象，無論添加多少次， 最終只會保留一個該對象（的引用）， 并且，保留的是第一次添加的那一個

*/

// student.courses.add(null);

student.courses.add(cr);

}

}

}

st.testForEachForSet(student);

}

public void testForEachForSet(Student student) {

// 打印輸出，學生所選的課程！

//System.out.println("共選擇了：" + student.courses.size() + "門課程！");

for (Course cr : student.courses) {

System.out.println("選擇了課程：" + cr.id + ":" + cr.name);

}

}

}