上界萬用字元和下界萬用字元
阿新 • • 發佈:2022-12-12
public static void main(String[] args) { /* Object:超級父類! User:使用者類! Student、Teacher:是User的子類! 上界萬用字元:無法進行資料儲存的! ? extends User:?是User的子類! --> Teacher或者Student 泛型:引數化型別! --> 只能傳遞一個數據型別! 取值:可以使用同一個型別將資料提取出來! */ List<? extends User> users = new ArrayList<>(10); users.add(null); //取值 User user = users.get(0); System.out.println(user); /* 下界萬用字元 ? super User:是User的父類! ? 是Object! ? super Student: */ List<? super User> users2 = new ArrayList<>(10); users2.add(new Student("123", "123")); users2.add(new Teacher("321", "321")); //取值 Object object = users2.get(0); if (object instanceof Student) { Student student = (Student) object; System.out.println(student); } else if (object instanceof Teacher) { Teacher teacher = (Teacher) object; System.out.println(teacher); } }