Conversions from a Java array of nested classes to a scala array

Question

Say I have the following class defined in java:

public class A
{
  public class B
  {
  }

  public B[] someFunc() {...}
}

And I am trying to access it in scala as follows:

val array: Array[A#B] = a.someFunc()

The compiler gives me the following warning:

*type mismatch; found : Array[a.B] required: Array[A#B] Note: a.B <: A#B, but class Array is invariant in type T. You may wish to investigate a wildcard type such as `_ <: A#B*

I am not sure of the correct syntax I should use to get over this error. I tried using the following but it will not compile:

val array: Array[T <: A#B] = a.someFunc()

But I have found away to get over the problem by passing it to a function:

def test[T <: A#B](array: Array[T]) = ...
test(a.someFunc())

which compiles fine.

How would I achieve the correct type assignment without having to define this test function?

Thanks

Des

gourlaysama · Accepted Answer · 2013-10-29 14:41:21Z

1

Your B inner class is not marked static, which means, from the scala point of view, it is not a member of the companion object (i.e. a static member) A#B but it is a member of the instantiated object itself a.B.

So you should just declare your value this way:

val array: Array[a.B] = a.someFunc()

Or let type inference do it:

val array = a.someFunc() // typed as Array[a.B]

Edit: in case you do not have a reference to a lying around, usually you can just upcast a.B to A#B. Array is invariant, so that won't work, but there is a ruse: wrap it in something covariant.

val array: IndexedSeq[A#B] = a.someFunc()

You can use that one like an array, it doesn't actually convert anything (it is a WrappedArray), and you can call toArray on it if you really want to get an Array[A#B].

That is just a trick to avoid doing a.someFunc().asInstanceOf[Array[A#B]], but of course you could directly do that.

edited Oct 29, 2013 at 14:41

answered Oct 29, 2013 at 13:19

gourlaysama

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3 Comments

user79074 Over a year ago

The problem with that solution is you need a reference to a specific instance of an A object. And this is not always going to be feasible. Say I want to define a class or a function to operate on such a type when I have no such instance. As I showed in my original question there is a way to achieve what I want by defining a function. I believe we are just missing the correct syntax to do this without having to define said function.

gourlaysama Over a year ago

you are right, you need an instance of A. The real problem is that path-dependent types are just compiler fiction; at runtime, everything is A#B. But there is no direct way to tell the compiler "I know what I am doing, this is safe" except, of course, a cast. See edit.

Ed Staub Over a year ago

There are a few areas of Scala that are definitely cringe-worthy; this (path-dependent typing of instances of non-static inner classes) seems to be one of them.

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