A function with generic return type

Question

Is it possible to have a function that returns a generic type? Something like:

fun <T> doSomething() : T {
    when {
        T is Boolean -> somethingThatReturnsBoolean()
        T is Int -> somethingThatReturnsInt()
        else -> throw Exception("Unhandled return type")
    }
}

Answer 1

You can use reified type to capture its class literal, like this:

inline fun <reified T> doSomething() : T {
    return when (T::class.java) {
        Boolean::class.java -> somethingThatReturnsBoolean() as T
        Int::class.java -> somethingThatReturnsInt() as T
        else -> throw Exception("Unhandled return type")
    }
}

But you also must convince the compiler that T is Boolean or T is Int, as in the example, with unchecked cast.

reified makes real T's type accessible, but works only inside inline functions. If you want a regular function, you can try:

inline fun <reified T> doSomething() : T = doSomething(T::class.java)
fun <T> doSomething(klass: Class<T>): T {
    return when (klass) { ... }
}