为什么我无法加入到已选课程中，courses中没有添加到课程

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> courseToSelect;

public SetTest(){

courseToSelect=new ArrayList<Course>();

}

public void testAdd(){

// 创建一个课程对象，并通过调用add方法，添加调用add方法，添加到备选课程;List中

Course cr1=new Course("1.","数据结构"); 

courseToSelect.add(cr1);

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

// System.out.println(temp.id+temp.name);

Course cr2=new Course("2.","C语言");

courseToSelect.add(1,cr2);

Course temp2=(Course) courseToSelect.get(1);

// System.out.println(temp2.id+temp2.name);

Course[] cr3={new Course("3.","离散数学"),new Course("4.","汇编语言")};

courseToSelect.addAll(Arrays.asList(cr3));

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

Course temp4=(Course) courseToSelect.get(3);

// System.out.println(temp3.id+temp3.name+temp4.id+temp4.name);

Course[] cr4={new Course("5.","算法"),new Course("6.","大学英语")};

courseToSelect.addAll(0,Arrays.asList(cr4));

Course temp5=(Course) courseToSelect.get(4);

Course temp6=(Course) courseToSelect.get(5);

// System.out.println(temp5.id+temp5.name+temp6.id+temp6.name);

}

public void foreach(){

System.out.println("foreach");

for(Course cr:courseToSelect){

System.out.println(cr.id+cr.name);

}

}

public static void main(String[] args) {

// TODO Auto-generated method stub

SetTest st=new SetTest();

st.testAdd();

st.foreach();

Student student=new Student("1","小米");

System.out.println("欢迎"+student.name+"选课");

Scanner console=new Scanner(System.in);

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

System.out.println("输入课程ID");

String courseId=console.next();

for(Course cr:st.courseToSelect){

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

student.courses.add(cr);

}

}

}

st.tesx(student);

System.out.println(student.courses.size());

}

public void tesx(Student student){

for(Course cr:student.courses){

System.out.println(cr.id+cr.name);

}

}

}

//学生类

package com.imooc.collection;

import java.util.HashSet;

import java.util.Set;

public class Student {

    public String id;

    public String name;

    public Set<Course> courses;

    public Student(String id, String name) {

        this.id = id;

        this.name = name;

        this.courses = new HashSet<Course>();

    }

}

//Course 类

package com.imooc.collection;

public class Course {

    public String id;

    public String name;

    public Course(String id, String name) {

        this.id = id ;

        this.name = name;

    }

    public Course() {

    }

}