I've problems writing a test implementation for an...

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[iex]

I've problems writing a test implementation for an interface:

interface Preferences {
    // ...
    fun <T> putObject(key: PreferencesKey, model: T?)
    fun <T> getObject(key: PreferencesKey, clazz: Class<T>): T?
}

Force returning a specific object, for unit testing

class PreferencesReturningObject<T>(val obj: T): Preferences {
    override fun <T> putObject(key: PreferencesKey, model: T?) {}
    override fun <T> getObject(key: PreferencesKey, clazz: Class<T>): T? = obj
}

How can I write this? (Without using Mockito or similar)
Edit: It's "fixed" by writing obj as T (the class type parameter actually doesn't make sense, it can be just Any) but wonder if there's a better way

[Brian Saltz Jr]

Did you intend to put the <T> on the Preferences declaration? I.e.:

interface Preferences<T>

What’s happening in your test implementation is that the <T> in PreferencesReturningObject<T> is being hidden by the functions’ <T> declaration, so the <T> from the class is not the same as the <T> being returned by the function.

Put differently, with your current code, this would compile, but fail at runtime:

val prefs = PreferencesReturningObject<String>("hello")
val number = prefs.getObject<Int>(PreferencesKey("mykey"), Int::class.java)

If the <T> on the functions is supposed to be different per-key, then yes, the way you’ve mocked it out is the best way of doing it without using a mocking library. However, if you’re expecting every preference value to have the same type, you should move <T> to the Preferences interface and remove the declaration from the functions.