java关于ArrayList()的简单问题

    package Hope;
    import java.util.ArrayList;
    import java.util.List;
    public class ListTest {
    public List coursesToSelec;

    /* public ListTest(){
    this.coursesToSelect = new ArrayList();
    }
    */
    coursesToSelect = new ArrayList(); //把上面的内容注释掉，换成这段代码为什么不行？

    public void addCourse(){
        Course cr1 = new Course("离散数学",1);
        coursesToSelect.add(cr1);
        Course temp = (Course) coursesToSelect.get(0);
        System.out.println(temp.name+" "+temp.number);

    }      

    public static void main(String[] args){
        ListTest a = new ListTest();
        a.addCourse();
    }

    }

    package Hope;
    public class Course {
    String name;
    int number;
    public Course(String name,int number){
    this.name = name;
    this.number = number;
    }

    }