Designing classes and interfaces with generic types

Question

I have the following interface that maps and accepts as a parameter a InsectTypesEntity and return a object of a InsectDataModel and another that returns a List<InsectDataModel>

I am trying to do this with generics, as I would like to practice this.

interface InsectInteractorMapper<T> {
    fun map(insectTypesEntity: T): T
    fun map(cursor: Cursor): List<T>
}

Without the generics it would be something like this:

interface InsectInteractorMapper<InsectTypesEntity> {
    fun map(insectTypesEntity: InsectTypesEntity): InsectDataModel
    fun map(cursor: Cursor): List<InsectDataModel>
}

I am trying to get my class that implements this using the generic version of the interface, however, I get many errors related:

1) Return type is 'insectDataModel' which is not a subtype of overridden
public abstract fun map(insectTypesEntity: InsectTypesEntity): InsectTypeEntity defined in InsectInteractorMapper

2) Return type is 'List<InsectDataModel>' which is not a subtype of overridden
public abstract fun map(cursor: Cursor): List<InsectTypesEntity> defined in InsectInteractorMapper

The class that implements the interface

class InsectInteractorMapperImp: InsectInteractorMapper<InsectTypesEntity> {
    override fun map(insectTypesEntity: InsectTypesEntity): InsectDataModel {

        return InsectDataModel(
                insectTypesEntity.friendlyName,
                insectTypesEntity.scientificName,
                insectTypesEntity.classification,
                insectTypesEntity.imageAsset,
                insectTypesEntity.dangerLevel)
    }

    override fun map(cursor: Cursor): List<InsectDataModel> {
        val insectDataModelList: MutableList<InsectDataModel> = mutableListOf()

        cursor.moveToFirst()
        while(cursor.moveToNext()) {
            InsectDataModel().let {
                it.friendlyName = cursor.getString(cursor.getColumnIndexOrThrow(InsectContract.COLUMN_FRIENDLY_NAME))
                it.scientificName = cursor.getString(cursor.getColumnIndexOrThrow(InsectContract.COLUMN_SCIENTIFIC_NAME))
                it.dangerLevel = cursor.getInt(cursor.getColumnIndexOrThrow(InsectContract.COLUMN_DANGER_LEVEL))

                insectDataModelList.add(it)
            }
        }

        cursor.close()
        return insectDataModelList.toList()
    }
}

What would be the best way to get this working correctly with generics?

many thanks for any suggestions,

==== UPDATE Modified interface for in/out variance:

interface InsectInteractorMapper<in E, out M> {
    fun map(insectTypesEntity: E): M
    fun map(cursor: Cursor): List<M>
}

However, I am getting a warning when I try and use the interface:

unchecked assignment java.util.List to java.util.List<InsectDataModel> Reason insectDataModelMapper has raw type so result of map will be erased 

When I use it like this:

insectInteractorMapper = new InsectInteractorMapperImp();
insectDataModelList = insectInteractorMapper.map(cursor);

Answer 1

Since you want a in type and an out type you need to declare those like so:

interface InsectInteractorMapper<in T1, out T2> {
    fun map(insectTypesEntity: T1): T2
    fun map(cursor: Cursor): List<T2>
}

Then your code will work