错误package imooc.collection_map_demo;
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(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);
}
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);
}
}
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 接收输入ID
Scanner console = new Scanner(System.in);
for(int i =0 ;i<0;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);
}
}
}Exception in thread "main" java.lang.NullPointerException at imooc.collection_map_demo.SetTest.testForEachForSet(SetTest.java:93) at imooc.collection_map_demo.SetTest.main(SetTest.java:89)
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