Help on Scala/Java integration

> TreePath tp = new TreePath(book.getPath());
> System.out.println(tp);
> ---------
> Result: [root, Twain, Huck]

`book.getPath()` returns `TreeNode[]`. The constructor of `TreePath`
takes `Object[]`. Now, for javac, these types are compatible and it
covariantly casts the array of `TreeNode` to an array of `Object`.

> ----- in Scala ---------------------------------------
> val tp = new TreePath(book.getPath())
> println(tp)
> ---------
> Result: [[Ljavax.swing.tree.TreeNode;@b5f53a]

For Scala, those types are not compatible such that it chooses the
constructor of `TreePath` taking an `Object`. (In Scala, arrays are
invariant.) There are two ways to address that:

1) tp.asInstanceOf[Array[Object]] (bad)
2) tp map { x => x: Object } (better)

Hi Lars,

Thank you for answering.

The first suggestion, using an explicit cast worked fine.

The second option, using the map doesn't compile ("value map is not a
member of javax.swing.tree.TreePath").

I'm curious... why is the second version better? Seems worse, as there
will be another val to receive the map result or change the original
val to var...

Thanks,

Marcos

On Jan 24, 7:24 am, Lars Hupel wrote:
> >    TreePath tp = new TreePath(book.getPath());
> >    System.out.println(tp);
> > ---------
> > Result:   [root, Twain, Huck]
>
> `book.getPath()` returns `TreeNode[]`. The constructor of `TreePath`
> takes `Object[]`. Now, for javac, these types are compatible and it
> covariantly casts the array of `TreeNode` to an array of `Object`.
>
> > ----- in Scala ---------------------------------------
> >            val tp = new TreePath(book.getPath())
> >            println(tp)
> > ---------
> > Result:    [[Ljavax.swing.tree.TreeNode;@b5f53a]
>
> For Scala, those types are not compatible such that it chooses the
> constructor of `TreePath` taking an `Object`. (In Scala, arrays are
> invariant.) There are two ways to address that:
>
> 1) tp.asInstanceOf[Array[Object]] (bad)
> 2) tp map { x => x: Object } (better)

> The second option, using the map doesn't compile ("value map is not a
> member of javax.swing.tree.TreePath").

I made a mistake in writing it down.

>> 2) tp map { x => x: Object } (better)

It should read

new TreePath(book.getPath() map { x => ...

> I'm curious... why is the second version better? Seems worse, as there
> will be another val to receive the map result or change the original
> val to var...

I guess casting would be acceptable here, but casts circumvent the type
system and might introduce unsoundness into your program.