Is intersection casting possible in Kotlin?

Question

I have a method in Java like so:

public <T extends A & B> methodName(T arg, ...)

where A is a class and B is an interface.

In my kotlin class, I have another variable of type C, and I wish to achieve the following:

if (variable is A && variable is B) {
    methodName(variable, ...)
} else {
    // do something else
}

Is it possible to properly cast variable so that it may be used as an argument without errors?

Currently, the variable has a setter method, so smart casting isn't available. However, I have also tested it with a local val and the value is inferred to have type Any which doesn't help.

Answer 1

Kotlin does not support intersection types. This causes variable to be smart cast to Any, because that is the common ancestor of A and B.

However, Kotlin does support generic type constraints. You can use this to constrain a type parameter to one or more types. This can be used on both methods and classes. This is the syntax for functions (the equivalent of your methodName in Kotlin):

fun <T> methodName(arg: T)
    where T : A,
          T : B {
    ....
}

You can use this to get around your problem by creating a class which extends both A and B, and then delegates the implementation of these types to your object. Like this:

class AandB<T>(val t: T) : A by t, B by t
    where T : A,
          T : B

You can now call methodName by changing your if-test to check if it is a AandB<*>:

if (variable is AandB<*>) {
    methodName(variable, ...)
}

You do need to wrap variable in a AandB somewhere though. I don't think you can do it if you don't have the type information for variable available anywhere.

Note: The AandB class does not implement hashCode, equals or toString. You could implement them to delegate to t's implementation.

Note 2: This only works if A and B are interfaces. You can not delegate to a class.