如何在另一个有界类型上定义方法有界类型？

Question

I want to define a method that defines its types like this:

List<R> toList(JsArray<T> array)

such that T is bounded such that it is both:

• T extends SomeClass , and
• T extends R

I've tried things like this, to no avail:

<R, T extends R & SomeClass> List<R> toList(JsArray<T> array)

(For the curious, this is to be able to use GWT Overlay Types with Interfaces )

Answer 1

How about this?

<T extends SomeClass> List<? super T> toList(JsArray<T> array)

EDIT:

I think I see your use case. From comments, you want to specify the return type so you don't have an unchecked cast.

But you don't actually pass anything in to tell the method what to create. Since I could always call toList() with R = T , the only thing you could do that always works (for all R super T ) is to return a List<T> , which makes the R parameter unnecessary... but that's not what you want.

You have to pass something in to tell the method what kind of object to instantiate. Usually we could to that like:

<R, T extends SomeClass> List<R> toList(JsArray<T> array, Class<R> cls)

and call it like

toList(arrayIHave, WhatIWant.class)

Now this will work, but you will complain that it has error conditions, because the " T extends R " constraint isn't captured. But really, even if you add the constraint in, you'll still have error conditions.

I can use an arbitrary interface for R by making a T that extends SomeClass and implements R . There is no way you can meaningfully instantiate an arbitrary interface, so for many R,T pairs that satisfy your constraints, you will have to return null or throw an exception or something.

No doubt there is some rule that dictates which R you will return, but there is no way to even think about specifying generic constraints that capture that rule, so there will be an unchecked conversion somewhere. If you use a signature like this, at least you can put it inside the method instead of having warnings or annotations everywhere you use it.

Answer 2

I fear this does not work. The correct syntax would be

<R, T extends R & SomeClass> List<R> toList(JsArray<T> array)

But this will give you the following error message

Cannot specify any additional bound SomeClass when first bound is a type parameter

I do not see how you could make it work to enforce both constraints.