`interface Foo<T> { fun bar(x: Any): Boolean...

# announcements

[Marian Schubert]

interface Foo<T> { fun bar(x: Any): Boolean { ... } } - is there some way to return true from bar if x is instance of T?

[streetsofboston]

An instance/object can never have the type T?. It will have the type T or its value is null.

[streetsofboston]

Hmm… sorry, the last ? is your question-mark 🙂

[Dominaezzz]

Not without reification.

[streetsofboston]

^^^ What Dominic said, and that won’t work with class and interface definitions. Reification works only with function definitions and extension-properties

[Dominaezzz]

interface Foo<T> {
    inline fun <reified T> bar(x: Any): Boolean {
        return x is T
    }
}

doesn't work, as @streetsofboston said.


next best thing is

inline fun <reified T> Foo<T>.bar(x: Any): Boolean {
     return x is T
}

[Dominaezzz]

Do you actually need the polymorphism?

[Marian Schubert]

well what I really want is something like:

interface Handler<C, R> { fun handle(c: C): R }

then I want Spring to inject all classes which implement that interface, and loop over them and check which class actually handles x's class, and then call foundHandler.handle(x)

[Marian Schubert]

so I thought I'll add doesHandle method into that interface which will do something like x is C

[streetsofboston]

If you need to put the generic C parameter in the interface definition, x is C won’t work. You can’t reify it.
But, you could examine the class of `x`: x::class.

interface Handler<C, R> {
    val classOfC: KClass<out C>
    ...
}
...
class TestHandler : Handler<InputData, OutputData> {
    override val classOfC = InputData::class
    ...
}

And use classOfC to check wether something is of the desired type in the other overridden methods of TestHandler.

Like Dominic said, if you don’t need polymorphism, use a reified generic type parameter on an extension function for Handler<C, R>