为何会越界异常

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>();

}

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(1); 

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

// coursesToSelect.add(cr1);

// Course temp0 = (Course) coursesToSelect.get(0);

// 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(3);

Course temp4 = (Course) coursesToSelect.get(4);

// System.out.println("添加了2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);

}

public void testForEach() {

System.out.println("有以下课程待选（通过for each返回）：");

for(Object obj:coursesToSelect) {

Course cr = (Course) obj;

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

}

}

public static void main(String[] args) {

// TODO Auto-generated method stub

SetTest st = new SetTest();

st.testAdd();

st.testForEach();

//创建一个新的学生对象

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，用来接收用键盘输入的课程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);

}

}

}

st.testForEachForSet(student);

}

public void testForEachForSet(Student student) {

//打印输出学生所选课程

for(Course cr : student.courses) {

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

}

}

}