英文:
Why cannot assign a subtype value to a generic type?
问题
可以有人解释一下：
class Test<T extends BaseDao>{
void someMethod(){
T inst = new Dao(); // 需要类型 T，但提供了 Dao
T inst2 = new BaseDao(); // 需要类型 T，但提供了 BaseDao
}
}
class Dao extends BaseDao{}
我们将 T 类型定义为 BaseDao 的子类型。为什么不能将 Dao 或 BaseDao 类型赋值给 T 类型呢？
英文:
Could anybody explain:
class Test<T extends BaseDao>{
void someMethod(){
T inst = new Dao(); // required type T provided Dao
T inst2 = new BaseDao(); // required type T provided BaseDao
}
}
class Dao extends BaseDao{}
We have T type as a subtype of BaseDao.
Why cannot assign Dao or BaseDao types to T type?
答案1
得分: 0
T可以是Dao的一个实例，但是你不能像这样将Dao的新实例分配给泛型类型T，因为T可能是指向任何扩展BaseDao的类。
如果你需要有Dao的某个特定实例，你应该这样做：
Dao inst = new Dao()
如果相反地，你确实想要一个特定的实例，但又不想知道具体的类，你可以将inst1和inst2作为参数传递，或者传递一个字符串并根据名称创建一个实例，还有许多其他可能性。
简而言之，T inst = new Dao();这样做是没有意义的，因为它与泛型的目的冲突。
英文:
T can be an instance of Dao, but you cannot assign a new istance of Dao to the generic type T like this because T could refer to any class extending BaseDao.
If you need to have some specific instance of Dao you should do
Dao inst = new Dao()
If instead you do want a specific instance, but you don't want to know the specific class, you could pass inst1 and inst2 as parameters, or pass a string and create an instance by name, or many other possibility.
In short, T inst = new Dao(); makes no sense because it collides with the purpose of generics.
答案2
得分: 0
Imagine
Test<MySpecialDao> specialDao = new Test<>();
Now, for a call like `specialDao.someMethod()`,
void someMethod(){
T inst = new Dao(); // required type T, provided Dao
T inst2 = new BaseDao(); // required type T, provided BaseDao
}
effectively becomes
void someMethod(){
MySpecialDao inst = new Dao(); // required type MySpecialDao, provided Dao
MySpecialDao inst2 = new BaseDao(); // required type MySpecialDao, provided BaseDao
}
and of course, both `Dao` and `BaseDao` aren't subtypes of `MySpecialDao`, probably supertypes.
I'd change to
void someMethod(){
Dao inst = new Dao();
BaseDao inst2 = new BaseDao();
}
hoping that this change doesn't break your intended logic.
英文:
Imagine
Test<MySpecialDao> specialDao = new Test<>();
Now, for a call like specialDao.someMethod(),
void someMethod(){
T inst = new Dao(); // required type T, provided Dao
T inst2 = new BaseDao(); // required type T, provided BaseDao
}
effectively becomes
void someMethod(){
MySpecialDao inst = new Dao(); // required type MySpecialDao, provided Dao
MySpecialDao inst2 = new BaseDao(); // required type MySpecialDao, provided BaseDao
}
and of course, both Dao and BaseDao aren't subtypes of MySpecialDao, probably supertypes.
I'd change to
void someMethod(){
Dao inst = new Dao();
BaseDao inst2 = new BaseDao();
}
hoping that this change doesn't break your intended logic.
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