Given a rather large 3rd party interface, I need t...

# codereview

[arve]

Given a rather large 3rd party interface, I need to implement only a small handful methods. (for testing, i control the calling code)

interface Alphabet {
    fun a(): String
    fun b(): String
    // and many more
    fun z(): String
}

I thought I had a really bright idea, since the following is valid syntax:

class A : Alphabet by TODO("NOPE") {
    override fun a() = "a"
}

I was hoping the delegation would act as with actual objects, routing calls to the override if they were present, and throwing NotImplementedError otherwise. Unfortunately, the TODO() is evaluated on instantiation of A() and not during method call 😞

// Hoping for
A().a() == "a" // true
A().b() // NotImplementedError

// but got 
A() // NotImplementedError

Does anyone have an elegant way to partially implement an interface and default to NotImplementedError for the rest, without explicitly doing so for every single method?

[ephemient]

on JVM, https://docs.oracle.com/javase/8/docs/technotes/guides/reflection/proxy.html would be possible

[ephemient]

and on all platforms you can build similar statically with KSP

[ephemient]

but it would be easier to use an existing mocking library

[arve]

Yeah, seems like alternatives are Proxy, explicitly TODO ’ing all methods, and making a “wrapping” interface. I ended up doing the latter.

[arve]

e.g.

interface TinyAlphabet { fun a(): String }

and expecting that in the calling code instead.

[Daniel Pitts]

If it’s for testing, perhaps mocking is what you want. Mockk is what I usually use.

[Daniel Pitts]

Here is the MockK site