Create a generic class with an array of type Comparable<T> in kotlin?

Question

Inspired by Create generic 2D array in Kotlin, I have the following code that creates a generic class with an array of type T. However, once I add an upper bound, I get a compile error. Is there a way to do this?

//This code compiles:
class GenericClass<T> protected constructor(size : Int, arrayFactory: (Int) -> Array<T>) {
  companion object {
    inline fun <reified T> invoke(size : Int)
       = GenericClass(size, { size -> arrayOfNulls<T>(size) })
  }
  val array = arrayFactory(size)
}

//Compile errors:
class GenericClass<T : Comparator<T>> protected constructor(size : Int, arrayFactory: (Int) -> Array<T>) {
  companion object {
    inline fun <reified T : Comparator<T>> invoke(size : Int)
       = GenericClass(size, { size -> arrayOfNulls<T>(size) })
  }
  val array = arrayFactory(max)
}

The compile errors are:

Type parameter bound for T in constructor GenericClass>(size: Int, arrayFactory: (Int) -> Array) is not satisfied: inferred type T? is not a subtype of Comparator

miensol · Accepted Answer · 2016-07-11 05:16:22Z

4

The error message while misleading does hint that it's about nullability constraints of your type arguments and parameters. Change the GenericClass constructor to allow for null inside Array like so:

class GenericClass<T : Comparator<T>> 
  protected constructor(size : Int, arrayFactory: (Int) -> Array<T?>) {
    companion object {
        inline fun <reified T : Comparator<T>> invoke(size : Int)
            = GenericClass(size, { size -> arrayOfNulls<T>(size) })
    }
    val array = arrayFactory(size)
}

arrayOfNulls as the name suggest creates array of given size filled with nulls. Hence if you wish to use it the arrayFactory needs to accept it.

answered Jul 11, 2016 at 5:16

miensol

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