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

- is there some way to return true from bar if x is instance of T?

An instance/object can never have the type

T?

. It will have the type

T

or its value is

null

.

Not without reification.

Hmm… sorry, the last

?

is your question-mark 🙂

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
}

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)

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>