按照老师教的，为什么会报错？
错误如图所示：
14 回答
package com.imooc.collection;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
/**
* 备选课程类
* @author Administrator
*
*/
public class ListTest {
/**
* 用于存放备选课程的List
*/
public List coursesToSelect;
public ListTest() {
this.coursesToSelect = new ArrayList();
}
/**
* 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);
}
/**
* 取得List中的元素的方法
* @param args
*/
public void testGet() {
int size = coursesToSelect.size();
System.out.println("有如下课程待选：");
for(int i= 0 ; i < size; i++) {
Course cr = (Course) coursesToSelect.get(i);
System.out.println("课程：" + cr.id + ":" + cr.name);
}
}
/**
* 通过迭代器来遍历List
* @param args
*/
public void testIterator() {
//通过集合的iterator方法，取得迭代器的实例
Iterator<Course> it = coursesToSelect.iterator();
System.out.println("有如下课程待选(通过迭代器访问)：");
while(it.hasNext()) {
Course cr = it.next();
System.out.println("课程：" + cr.id + ":" + cr.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);
}
}
/**
* 修改List中的元素
* @param args
*/
public void testModify() {
coursesToSelect.set(4, new Course("7", "毛概"));
}
/**
* 删除List中的元素
* @param args
*/
public void testRemove() {
// Course cr = (Course) coursesToSelect.get(4);
System.out.println("即将删除4位置和5位置上的课程！");
Course[] courses = {(Course) coursesToSelect.get(4), (Course) coursesToSelect.get(5)};
coursesToSelect.removeAll(Arrays.asList(courses));
// coursesToSelect.remove(4);
System.out.println("成功删除课程！");
testForEach();
}
/**
* 往List中添加一些奇怪的东西
* @param args
*/
public void testType() {
System.out.println("能否往List中添加一些奇怪的东西呢！？");
coursesToSelect.add("我不是课程，我只是一个无辜的字符串！！");
}
public static void main( String[] args) {
ListTest lt = new ListTest();
lt.testAdd();
lt.testType();
lt.testForEach();
// lt.testGet();
// lt.testIterator();
// lt.testForEach();
// lt.testModify();
// lt.testForEach();
// lt.testRemove();
}
}
package com.imooc.collection;
import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
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(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，添加某一个对象
* 无论添加多少次，最终只会保留一个该对象的引用
* 并且保留的的是第一次添加的那个
*/
//student.courses.add(cr);
}
}
}
st.testForEachSet(student);
}
public void testForEachSet(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);
}
// TODO Auto-generated method stub
}
}
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