请教大神 虚心请教

这是测试页面

package uuu;

import java.util.Arrays;

import java.util.Iterator;

import java.util.List;

public class ListTest {

   private List courseToselect;

public List getCourseToselect() {

return courseToselect;

}

public void setCourseToselect(List courseToselect) {

this.courseToselect = courseToselect;

}

   public ListTest(){

  this.courseToselect = ArrayList();

   }

private List ArrayList() {

// TODO Auto-generated method stub

return null;

}

public void  listAdd(){

Course cr1 = new Course("01 ", "  JAVA");

courseToselect.add(cr1);

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

System.out.println("添加了课程： "+temp.getId()+temp.getName());

Course cr2 = new Course("02 ", "  C语言");

courseToselect.add(0, cr2);

Course temp2 = (Course)courseToselect.get(0);

System.out.println("添加了课程： "+temp2.getId()+temp2.getName());

Course cr3 = new Course("03 ", "  大学数学");

courseToselect.add( cr3);

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

System.out.println("添加了课程： "+temp3.getId()+temp3.getName());

Course cr4 = new Course("04 ", "  大学英语");

courseToselect.add(2, cr2);

Course temp4 = (Course)courseToselect.get(2);

System.out.println("添加了课程： "+temp4.getId()+temp4.getName());

Course as[] = {new Course("5 ", "  大学语文"),

new Course("6 ", "  毛概")};

courseToselect.addAll(3,Arrays.asList(as));

Course temp5 = (Course) courseToselect.get(3);

Course temp6 = (Course) courseToselect.get(4);

System.out.println("添加了课程： "+temp5.getId()+temp5.getName()

        +temp6.getId()+temp6.getName());

}

  public void getcourse(){

 Iterator it  = courseToselect.iterator();

 while (it.hasNext()) {

 Course course  = (Course) it.next();

System.out.println("添加了如下课程:");

System.out.println(course.getId()+":"+course.getName());

}

  }

   public static void main(String[] args) {

ListTest  lt = new ListTest();

lt.addtest();

}

}

在我所有的属性封装操作以后   调用lt.addtest();  为什么会报错

出现提示 是  Add cast to'lt'

变成  ((Object) lt).addtest();

请教大神  这一节封装   该怎么写 

学生类封装 ：

package uuu;

import java.util.HashSet;

import java.util.Set;

import asd.student;

public class Students {

   private String  id;

   private String  name;

   private Set  courses;

   public Students (String id, String name){

   this.id = id;

   this.name = name;

   this.courses = new HashSet();

   }

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public Set getCourses() {

return courses;

}

public void setCourses(Set courses) {

this.courses = courses;

}

}

课程类封装：

package uuu;

import java.util.List;

public class CourseList {

    private String  id;

    private String name;

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

    public CourseList(String id,String name){

    this.id = id;

    this.name = name;

    }

}

拜托诸位大神