hey everyone. I’ve got a question about interfaces...

# getting-started

[Oleg Shuliak]

hey everyone.
I’ve got a question about interfaces.
I can’t yet understand the best way of how to decide which concrete implementation of interface to use.
Let’s imagine, that I have ServiceInterface that has 2 implementations ConcreteService1 and ConcreteService2
It’ll have one function doWork() .
I need to use one implementation or another based on some property, that comes as a parameter. And I don’t understand where do I resolve which concrete class to use.

class SomeClass(
    val someService: ServiceInterface
) {
    fun someFunction(status: String) {
        when(status) {
            "ACTIVE" -> someService.doWork() // or even better to return the instance of service
            "INACTIVE" -> someService.doWork()
        }
    }
}

Could someone give a hint?

[andylamax]

Looks like you have the answer in your question.

I need to use one implementation or another based on some property

Since you need both implementations, you have to have them ready. Something in the lines of

class SomeClass(
    val service1: ServiceInterface,
    val service2: ServiceInterface
) {
    fun someFunction(status: String) {
        val service: ServiceInterface = when(status) {
            "ACTIVE" -> service1
            "INACTIVE" -> service2
        }

        service.doWork() // you can return this service if that what you wanted
    }
}

[Oleg Shuliak]

ok, that makes sense.
but if

val service1: ServiceInterface,
    val service2: ServiceInterface

are the same type of ServiceInterface , where the magic happens and service1 becomes ConcreteService1 ?

[PoisonedYouth]

You either use a dependency injection framework, which is providing the correct instance or you need a kind of factory, which returns the concrete service depending on some custom logic

[Oleg Shuliak]

ok, got it.
thanks a lot so explanation!