Kotlin put generic type object into specific hashmap

Question

I'm doing small library in Kotlin which handles presenter during configuration changed. I create presenter store class where I going to keep the presenters. I use generic class but I have problem but maybe I'll show you the code.

interface AbstractStore {
    fun clear()
    fun <V : AbstractView> put(key: String, presenter: Presenter<V>)
    fun <V : AbstractView> get(key: String): Presenter<V>? 
}

and implementation of Abstract Store

class PresenterStore : AbstractStore {

    private val mMap = HashMap<String, Presenter<AbstractView>>()

    override fun <V : AbstractView> put(key: String, presenter: Presenter<V>) {
        mMap[key] = presenter   // problem is here. I can't do this in that way.
    }

    override fun <V : AbstractView> get(key: String): Presenter<V>? {
        return mMap[key]
    }

    /**
     * Clears internal storage that Presenters are no longer used.
     */
    override fun clear() {
        mMap.forEach { (_, presenter) -> presenter.onDestory() }
        mMap.clear()
    }
}

I got problem with the put method. The message what I get:

Error:(12, 10) Type inference failed: Cannot infer type parameter V in operator inline fun <K, V> MutableMap<K, V#1 (type parameter of kotlin.collections.set)>.set(key: K, value: V#1): Unit
None of the following substitutions
receiver: MutableMap<String, Presenter<AbstractView>>  arguments: (String,Presenter<AbstractView>)
receiver: MutableMap<String, Presenter<V#2 (type parameter of com.xxxx.mvp.presenter.PresenterStore.put)>>  arguments: (String,Presenter<V#2>)
can be applied to
receiver: HashMap<String, Presenter<AbstractView>>  arguments: (String,Presenter<V#2>)

I don't get it what is going on but If I change it the method to

mMap[key] = presenter as Presenter<AbstractView> then works with warning

Unchecked cast: Presenter<V> to Presenter<AbstractView>

Is there any possibilities to keep method without casting? Why Kotlin doesn't know that Presenter is a generic class with subtype of AbstractView?

s1m0nw1 · Accepted Answer · 2018-02-23 20:54:11Z

1

Instead of making the two methods independently generic, use a class type parameter like so:

interface AbstractStore <V : AbstractView> {
    fun clear()
    fun put(key: String, presenter: Presenter<V>)
    fun get(key: String): Presenter<V>? 
}

Then, in the implementation, define a concrete type:

class PresenterStore : AbstractStore<ConcreteView> {

    override fun put(key: String, presenter: Presenter<ConcreteView>) {
        mMap[key] = presenter
    }

    override fun get(key: String): Presenter<ConcreteView>? {
        return mMap[key]
    }

    private val mMap = HashMap<String, Presenter<ConcreteView>>()
}

Now it works and is completely type safe.

answered Feb 23, 2018 at 20:54

s1m0nw1

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1 Comment

Michael Over a year ago

Thx. The simple the best.

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