报string异常

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);

}

}

private void testForEach() {

System.out.println("有如下课程待选(通过for each访问)：");

Course course=new Course();

for (Object obj : coursesToSelect) {

Course cr =(Course) obj;

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

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to com.imooc.collection.Course

at com.imooc.collection.ListTest.testGet(ListTest.java:127)

at com.imooc.collection.ListTest.main(ListTest.java:118)