How to cast a HashMap as a subclass in scala?

Question

I've created the following class:

class Foo[T] extends collection.mutable.HashMap[T, Int] {...}

(The class has some methods, but to reproduce this problem it doesn't need any.)

When I clone an instance of this class and try to use it as another instance of Foo, I get the following type error:

scala> val f = new Foo[String]
f: Foo[String] = Map()

scala> val anotherF: Foo[String] = f.clone
<console>:9: error: type mismatch;
found   : scala.collection.mutable.HashMap[String,Int]
required: Foo[String]

How do I cast the result of f.clone as an instance of Foo[String] ?

Update A commenter pointed out that I could override the clone method on HashMap. No objections to doing so, but it seems like I should be able to do so without reimplementing the actual cloning process.

Answer 1

The HashMap class returns HashMap objects, whether it is extended or not. If you want to change the return value, you have to extend MapLike as well.