是否可以将泛型类型扩展为Java中的多个类？

Question

I am trying to write a method in Java that uses a generic parameter and which I want to restrict to being exclusively either one of three possible classes (either Field , Method , or Constructor ).

I've tried the following header:

private static <T extends Field,Method,Constructor> T[] sort(T[] anArray)

But this way it ignores generic parameters of type Method or Constructor . Using the following also produces an error:

private static <T extends Field | Method | Constructor> T[] sort(T[] anArray)

Is it possible to do such a thing in Java ?

Answer 1

For your particular case (If you are talking about the Reflection specific classes), you are in luck.

You can use the AccessibleObject class.

private static <T extends AccessibleObject> T[] sort(T[] anArray)

You may also be interested in Member interface depending on your requirement.

Answer 2

Type-parameter bound cannot be defined using the or operator ( | ).

It's only possible to restrict a type-parameter T to extend a class and multiple interfaces, though, by using the & operator:

<T extends SomeClass & FirstInterface & SecondInterface>

Answer 3

No, but you could use a common interface/super class, just use Object and check inside the method or provide separate overloads.

Note that A & B would be possible, ie require that a generic type implements both types. You can't use or (ie A | B ) because the compiler can't/won't determine a common super type - if it even exists besides Object - and thus your implementation can't work on any of the types because when in doubt you'll get passed the wrong type.

Think about it: if you were allowed to declare T as T extends Field | Method T extends Field | Method , how would you use it inside the code? All you'd really know is that both types provide the methods of Object .

Generics are meant to remove the need for (potentially failing) casts in order to access a specific type's methods (ie the generic type's upper bound). Restricting parameter types comes as a consequence of this but is not a primary objective the way you seem to want to use it. That's what method overloads are for.

Edit: I just read Codebender's answer and that's what I mentioned in my first sentence: use a common interface or, as is the case here, super class. That's probably the best way to go.

Answer 4

Field , Constructor and Method each inherit from Member . You could just do:

private static <T extends Member> T[] sort(T[] anArray)

It wouldn't make sense to specify a type bound which could match one of several unrelated types. The whole point of generics is to give the compiler enough information to determine what methods would be available to the type T given that we don't know exactly what T will actually be. If it can be one of three things, how can the compiler possibly know which methods should be allowed?