为什么会显示空呢？

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;

private Scanner console;

public Student student ;

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(1, 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);

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

CoursesToSelect.addAll(Arrays.asList( cour));

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[] cour2= {new Course("5","高等数学"),new Course("6","大学英语")};

CoursesToSelect.addAll(2,Arrays.asList(cour2));

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 SetTest() {

// CoursesToSelect=new ArrayList<Cour>();

// console=new Scanner(System.in);

// }

/**

* 测试List的Contains方法

* @param args

*/

public void testListContains() {

//取得备选课程序列的第0个元素

Course course=CoursesToSelect.get(0);

//打印输出couresToSelect是否包含course对象

System.out.println("取得课程:"+course.name);

System.out.println("备选课程中是否包含课程:"+course.name+","+CoursesToSelect.contains(course));

//提示输入课程名称

System.out.println("请输入课程名称：");

String name=console.next();

//创建一个新的课程对象，ID和名称，于course对象完全一样

Course course2=new Course();

course2.name=name;

System.out.println("新创建课程:"+course2.name);

System.out.println("备选课程中是否包含课程:"+course2.name+","+CoursesToSelect.contains(course2));

}

//创建学生对象并选课

public void creatStudentAndSelectCours() {

//创建一个学生对象

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

console=new Scanner(System.in);

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

System.out.println("请输入课程ID:");

String courId=console.next();

for(Course cr:CoursesToSelect) {

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

student.courses.add(cr);

}

}

}

}

/**

* 测试Set的contains方法

* @param args

*/

public void testSetContains() {

//提示输入课程名称

System.out.println("请输入学生已选的课程名称");

String name=console.next();

//创建一个新的课程对象，ID和名称，于course对象完全一样

Course course2=new Course();

course2.name=name;

System.out.println("新创建课程:"+course2.name);

System.out.println("备选课程中是否包含课程:"+course2.name+","+student.Cour.contains(course2));

}

public static void main(String[] args) {

SetTest st=new SetTest();

st.testAdd();

//st.testListContains();

st.testForEach();

st.creatStudentAndSelectCours();

st.testSetContains();

}

public void testForeachForSet(Student student) {

//打印除数，学生所选的课程！

for(Course cr:student.Cour) {

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

}

}

}

Exception in thread "main" java.lang.NullPointerException

at com.imooc.collection.SetTest.testAdd(SetTest.java:17)

at com.imooc.collection.SetTest.main(SetTest.java:119)