为什么我程序输出不是无序的？？

package imooc.cn;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.Scanner;

public class Settest {

public List<Course>courseToselect;

public Settest(){

courseToselect= new ArrayList<Course>();

}

public void testadd(){

/*用于往coursetoselect中添加备选课程*/

      Course c1=new Course("1","数据结构");

      courseToselect.add(c1);

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

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

 Course c2=new Course("2","C语言");

 courseToselect.add(0, c2);

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

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

//以下方法会抛出数组下标越界异常  

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

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

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

Course temp4=(Course)courseToselect.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")};

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

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

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

//System.out.println("添加了两门课程："+temp5.id+":"+temp5.name+";"+temp6.id+":"+temp6.name);

}

public void foreach(){

System.out.println("foreach方法访问：");

for(Object obj:courseToselect){

Course c=(Course)obj;

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

}

}

public static void main(String[] args) {

// TODO Auto-generated method stub

     Settest st=new Settest();

     st.testadd();

     st.foreach();

     //创建学生

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

     System.out.println("欢迎"+xuesheng.name+"同学选课");

     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 c:st.courseToselect){

    if(c.id.equals(courseid)){

    xuesheng.courses.add(c);

    }

    }

     }

     System.out.println("共有多少门课程："+xuesheng.courses.size());

     //打印输出学生所选课程！

     st.foreachset(xuesheng);

}

public void foreachset(Student xuesheng){

for (Course c : xuesheng.courses) {

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

}

}

}