public?class?ListTest?{
????public?List?coursesToSelect;

????/**
?????*?備選課程類
?????*/
????public?ListTest()?{
????????this.coursesToSelect?=?new?ArrayList();
????}


????public?void?testAdd(){
????????Course?cr1?=?new?Course("1","數據結構");
????????coursesToSelect.add(cr1);
????????Course?temp?=?(Course)?coursesToSelect.get(0);
????????System.out.println("添加了課程："?+temp.id?+?":"+temp.name);

????????Course?cr2?=?new?Course("2","C語言");
????????coursesToSelect.add(0,cr2);
????????Course?temp2?=?(Course)coursesToSelect.get(1);
????????System.out.println("添加了課程："?+?temp2.id+":"?+temp2.name);
????}


????public?static?void?main(String[]?args)?{
????????ListTest?lt?=?new?ListTest();
????????lt.testAdd();
????}


}

為什么結果是null？