HashMap in scala.collection.mutable is invariant but immutable.HashMap is covariant, why?

Question

I want to extends a class with a val of mutable.HashMap[] like this:

class Father
class Son extends Father    

class C1{
  val m = new mutable.HashMap[Int, Father]() 
}

class C2 extends C1{
  override val m = new mutable.HashMap[Int, Son]()
}

And get an error:

Error:(19, 16) overriding value m in class C1 of type scala.collection.mutable.HashMap[Int,ScalaByExample.Father]; value m has incompatible type override val m = new mutable.HashMapInt, Son

I found that immutable.HashMap is covariant but mutable.HashMap is invariant. It works if replace mutable.HashMap with immutable.HashMap.

So my two questions are:

1. How can I make it works using mutable.HashMap?

2. Why did scala's author design HashMap like this?

Answer 1

Mutable maps are invariant because writing to them wouldn't be safe otherwise. Consider for example the following function:

def f(o: C1) {
    o.m(42) = new Father
}

This method is perfectly well-typed. But if you passed in an instance of C2 as the value for o, it'd break because a Map[Int, Son] is not allowed to contain Father objects. Therefore your definition of C2 is ill-typed.