Map of elements with variable generics

Question

There's this question for Java How to declare a map with variable generics?

I have the exact same problem. Is there a better/Scala way of solving it?

EDIT: I tried following the above answer, but without implementing an internal map.

private class SectionConversionMap
  extends HashMap[SectionSchema[_], (Iterable[HtmlRow]) => Option[Iterable[_]]]
{
  override def +[T, B1 >: ( Iterable[HtmlRow] ) =>
    Option[Iterable[T]]](kv: (SectionSchema[T], B1)):
  HashMap[SectionSchema[_], B1] = {
    val m = super.+(kv)
    m
  }
}

But my IDE keeps insisting that Expression of type HashMap[SectionSchema[_], Any] doesn't conform to expected type HashMap[SectionSchema[_], B1]

Does that really make a difference? – Anton Sep 01 '15 at 00:23 — Anton, Sep 01 '15 at 00:23
Well, let's go with the basics - immutable HashMap – Anton Sep 01 '15 at 00:32 — Anton, Sep 01 '15 at 00:32

score 3 · Answer 1 · answered Sep 01 '15 at 04:16

Don't extend standard collection types, it's just asking for trouble unless you know very well what you are doing. In this case the compiler will ensure all method signatures are at least as general as before, and your + isn't (note that in Binzi Cao's answer it doesn't override anything!). Just follow the source and keep the Map in a field.

Binzi Cao · Accepted Answer · 2015-09-01T01:57:33.897

Here is a simple example, maybe it can give you some hint:

import scala.collection.mutable._
class MyMap extends HashMap[Any, List[Any]] {
  def put[T, B <% T](key: T, value: List[B]) = {
    this += key -> value
  }
}
val b = new MyMap()
b.put("q", List("q"))
b.put(12, List(12))
b.put(122, List("23")) //not compile

The last line will not compile:

No implicit view available from String => Int.

It seems you want to ovrride the standard scala lib function, however, I guess, you could not change the method return types if you want to do a override. The below is the scala Map Method

@migration("`+` creates a new map. Use `+=` to add an element to this map and return that map itself.", "2.8.0")
  def + [B1 >: B] (kv: (A, B1)): Map[A, B1] = clone().asInstanceOf[Map[A, B1]] += kv

I did something similar in your code and it can compile now, hopefully it is what you needed or give you some clue:

import scala.collection.mutable._
type HtmlRow = String
type SectionSchema[T] = List[T]
private class SectionConversionMap extends HashMap[SectionSchema[_], (Iterable[_]) => Option[Iterable[_]]] {
   def +[T, B1 >: ( Iterable[HtmlRow] ) => Option[Iterable[T]]](kv: (SectionSchema[T], B1)):
       HashMap[SectionSchema[_], B1] = { 
         val m =  clone().asInstanceOf[HashMap[SectionSchema[_], B1]]
         m+=kv
       }   
}

Could you extend your answer to the case in my edit? Thank you. — Anton, Sep 01 '15 at 01:10
Note that this `def +` doesn't override the old one (you'd need `override` keyword otherwise). — Alexey Romanov, Sep 01 '15 at 04:33
@Binzi, without overriding, how do I know that the map will always contain correctly paired values - `[T, B1 >: ( Iterable[HtmlRow] ) => Option[Iterable[T]]](kv: (SectionSchema[T], B1))` and not of the more general signature? — Anton, Sep 02 '15 at 15:51

Map of elements with variable generics

2 Answers2