泛型加入课程对象为什么总报错

import java.util.*;

public class TestGeneric {

public List<Course> courses;

public void TestGeneric(){

courses=new ArrayList<Course>();

}

public void testAdd(){

Course c1=new Course("1","课程1");

courses.add(c1);

Course c2=new Course("2","课程2");

courses.add(c2);

}

courses.add(c1);为什么这行总报错