CODEWITHSUNDEEP
genericskotlinparameter-passinggeneric-type-argument
musooff
musooff

Reputation: 6882

How to pass a type to generic method in Kotlin?

I have a generic method like below

private fun <T> getSomething(): T {
    return "something" as T
}

How can I call this method with a variable T type?

val types = arrayListOf<Type>(String::class.java, Boolean::class.java)
types.forEach { type ->
    val something = getSomething<type>() // Unresolved reference: type
}

At runtime, I don't know what would be generic type T. I am getting the type from types and should pass it with generic getSomething method.

Use case

I want to call database which has several table. Example models are like this

class User{

}

class Student{

}

Since all the calling queries are basically same, I want to have generic method for calling database and get data. 

private fun <T> getData(model: String): List<T>?{
    return when(model){
        "user" -> getUsers()
        "student" -> getStudents()
        else -> null
    }
}

So when I call above method. Within my loop I want to pass Type as either User or Student.

val types = arrayListOf<Type>(User::class.java, Student::class.java)
types.forEach { type ->
    val data = getData<type>(type.javaClass.simpleName) // Unresolved reference: type in <type>
}

How can I achieve it.

Upvotes: 62

Views: 100680

Answers (3)

John Caron
John Caron

Reputation: 1387

If you want to restrict your types to something other than Any, you can create a parameterized DataType:

data class Datatype<T> {
    companion object {
        val USERS = Datatype<User>
        val STUDENTS = Datatype<Student>
        ...
...

And send that to your generic function:

fun <T> getData(datatype: Datatype<T>): List<T>? {
    return when(datatype) {
        Datatype.USERS -> getUsers() as List<T>
        Datatype.STUDENTS -> getStudents() as List<T>
        else -> null
    }
}

Upvotes: 0

Dmitry
Dmitry

Reputation: 344

I would stick to concrete types like

import kotlin.reflect.KClass

interface IBaseData
interface IDataTable<out T> where T : IBaseData
{
    fun getData(): List<T>
}

class User : IBaseData
class Student : IBaseData

class UserTable : IDataTable<User>
{
    override fun getData(): List<User>
    {
        return listOf(User())
    }
}

class StudentTable : IDataTable<Student>
{
    override fun getData(): List<Student>
    {
        return listOf(Student())
    }
}

inline fun <reified T: IBaseData> getDataTable() : IDataTable<T>?
{
    return when(T::class)
    {
        User::class -> UserTable() as IDataTable<T>
        Student::class -> StudentTable() as IDataTable<T>
        else -> null
    }
}

fun main()
{
    var user = getDataTable<User>()?.getData()
    var student = getDataTable<Student>()?.getData()
}

But still, it's an overhead, why not to use getUser() or getStudents() directly

Upvotes: 3

JB Nizet
JB Nizet

Reputation: 692181

Here's a complete example:

import kotlin.reflect.KClass

data class User(val name: String)
data class Student(val name: String)

fun getUsers(): List<User> = listOf(User("JB"))
fun getStudents(): List<Student> = listOf(Student("Claire"))

fun <T: Any> getData(clazz: KClass<T>): List<T>? {
    return when(clazz) {
        User::class -> getUsers() as List<T>
        Student::class -> getStudents()  as List<T>
        else -> null
    }
}

fun main(args: Array<String>) {
    val types = listOf(User::class, Student::class)
    types.forEach { type ->
        val data = getData(type)
        println(data)
    }
}

Upvotes: 74

Related Questions