Cannot create a tuple containing a null in Scala

Question

The following code does not compile:

var next: (A, A) = (null, n)

Error:

error: type mismatch;
found   : Null(null)
required: A
var next: (A, A) = (null, n)

Also var next: (A, A) = ((null: A), n) fails with the same error.

Somehow I think it should compile.

I'm currently using the following code which seems to work:

var next: (A, A) = (null.asInstanceOf[A], n)

Why doesn't it work? Bug or feature?

Edit

After reading didiers answer the problem is obvious. I missed that null can only be assigned to AnyRef types. For my problem I choose to make the tuple (n,n) and have a boolean flag whether the first entry is valid. Depending on the application Option might be a better solution.

Answer 1

Probably because your A parameter type is not guaranteed to be <: AnyRef. It should if you intend to pass null. If so, I would say feature

Edit. See Daniel's comment below, the solution is >: Null, See AgileSteel answer too.

Answer 2

I'm not sure what you are trying to archive by using nulls (except more NPE at runtime), but I can recommend you not to use null in your Scala code. Scala has Option type that can be used instead of null. So in your case it would look like this:

var next: (Option[A], A) = (None, n)

I find this code much more clear and it's big advantage, is that tuple type tells other people (and compiler) that it's first element is not some A, but it either Some[A] or None.

If you need to integrate with existing Java code that uses null, that you can wrap objects that come from Java side in option as soon as possible. You can make it like this:

var next: (Option[A], A) = (Option(getAFromSomeMethodThatCanReturnNull()), n)

This answer probably does not answer your original question, but I hope it would be somehow helpful.