CODEWITHSUNDEEP
genericskotlin
Martin Eden
Martin Eden

Reputation: 6262

In Kotlin, how can I instantiate a generic class using a native java parameter?

I am exploring using BDB JE (Berkeley DB Java Edition) with Kotlin. Knowledge of BDB is not necessary to answer this question.

BDB has a method that looks like this:

store.getPrimaryIndex(Int::class.java, "Int", Person::class.java, "Person")

I want to do things generically, so I wrote this

inline fun <reified TModel : Any, reified TKey : Any> getIndex() = 
    return store.getPrimaryIndex(TKey::class.java, TKey::class.simpleName, TModel::class.java, Model::class.simpleName)

So far so good. I now want to pass this index object to a class, which looks like this:

class ModelStore<TModel, TKey>(index : PrimaryIndex<TKey, TModel>) {
    private val index = index

    fun get(key : TKey): TModel = index.get(key)
    fun put(model : TModel) = index.put(model)
}

But if I try and pass the output from getIndex<User, Int>() to ModelStore<User, Int> I get the following error:

Type inference failed: Expected type mismatch: Inferred type is PrimaryIndex<TModel!, TKey!>! but PrimaryIndex<TModel, TKey> was expected.

My question: Can I pass the the index to the ModelStore? How do I convince the type-system that this is kosher?

Upvotes: 1

Views: 525

Answers (1)

voddan
voddan

Reputation: 33749

The simplest workaround is to cast the types:

ModelStore<User, Int>(getIndex<User, Int>() as PrimaryIndex<User, Int>)

Also I would expect it to work without specifying the types:

ModelStore(getIndex<User, Int>())

Upvotes: 2

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