Set<String> keySet，这时ketSet里面的元素全部变成String类型吗
public void testKeySet(){
Set<String > keySet = student.keySet(); //此时的keySet是字符串类型？？？那键值对呢，学生对应课程怎么算呢？
System.out.println("总共有："+student.size()+"个学生！");
for (String stuId:keySet){
Student st = student.get(stuId);
if (st != null)
System.out.println("学生："+st.name);
}
}
