Help with Scala/Java Integration

Hi Marcos,

in Scala `Array[TreeNode]` is no subtype of `Array[AnyRef]` which is
the equivalent of `Object[]` in Java. That means that the
`TreeNode(Object[])` constructor can't be selected. To make it
eligible you have create an `Array[AnyRef]` directly (or cast to it).

Using `new TreePath(path.asInstanceOf[Array[AnyRef]])` should work.

The underlying cause is that in Java arrays are 'covariant. In this
case that means that in Java `Object[]` is a superclass of
`TreeNode[]` and you can always pass a `TreeNode[]` value when an
`Object[]` value is expected.

Because Array's covariance is potentially unsafe because arrays are
mutable, in Scala arrays are invariant. That means that
`Array[TreeNode]` is no subtype of `Array[AnyRef]`.

HTH
Johannes

On Tue, Jan 24, 2012 at 3:54 AM, Marcos Ackel wrote:
> Hello,
>
> When using (Java) swing and Scala together I found this difference and
> don't know how to solve it. Can anyone please help?
>
> Why is the Scala result different and how the Scala code should be
> modified to give the same result as in Java?
>
> TreePath class has 2 constructors: TreePath(Object)   and
> TreePath(Object[])
>
> It seems that the former constructor is being used in Scala (line 9),
> but the val "path" is an array. (Doesn't help coding
>  ... new TreePath(path.asInstanceOf[Array[TreeNode]])
> nor
>  val path : Array[TreeNode] = author.getPath()
> )
>
> Thank in advance,
>
> Marcos
>
> ----  in Java -----------------------
>        DefaultMutableTreeNode root = new DefaultMutableTreeNode();
>        root.setUserObject("root");
>        DefaultMutableTreeNode author = new DefaultMutableTreeNode();
>        author.setUserObject("Twain");
>        root.add(author);
>
>        DefaultMutableTreeNode book = new DefaultMutableTreeNode();
>        book.setUserObject("Huck");
>        TreeNode[] path = author.getPath();
>        TreePath tp = new TreePath(path);
>        TreePath tp2 = tp.pathByAddingChild(book);
>        System.out.println(tp2.getPathCount());
>        for (int i = 0; i < tp2.getPathCount(); i++)
>                System.out.println("p" + i + " " + tp2.getPathComponent(i));
> -----------
> Result:
>
> 3
> p0 root
> p1 Twain
> p2 Huck
> ----------------------- In Scala
> --------------------------------------------
> 1               val root = new DefaultMutableTreeNode()
> 2               root.setUserObject("root")
> 3               val author = new DefaultMutableTreeNode()
> 4               author.setUserObject("author")
> 5               root.add(author)
>
> 6               val book = new DefaultMutableTreeNode()
> 7               book.setUserObject("Huck")
> 8               val path = author.getPath()
> 9               val tp = new TreePath(path)
> 10              val tp2 : TreePath = tp.pathByAddingChild(book)
> 11              println(tp2.getPathCount())
> 12              for ( i <- 0 until tp2.getPathCount())
> 13                      println("p" + i + " " + tp2.getPathComponent(i))
> ---------------
> Result:
>
> 2
> p0 [Ljavax.swing.tree.TreeNode;@6f7ce9
> p1 Huck
>
> ---------------------------------------

Hi Johannes,

using your suggestion
`new TreePath(path.asInstanceOf[Array[AnyRef]])`

worked fine. I've _many_ things to learn in Scala.... Thank you very
much!

Marcos

On Jan 24, 5:08 am, Johannes Rudolph
wrote:
> Hi Marcos,
>
> in Scala `Array[TreeNode]` is no subtype of `Array[AnyRef]` which is
> the equivalent of `Object[]` in Java. That means that the
> `TreeNode(Object[])` constructor can't be selected. To make it
> eligible you have create an `Array[AnyRef]` directly (or cast to it).
>
> Using `new TreePath(path.asInstanceOf[Array[AnyRef]])` should work.
>
> The underlying cause is that in Java arrays are 'covariant. In this
> case that means that in Java `Object[]` is a superclass of
> `TreeNode[]` and you can always pass a `TreeNode[]` value when an
> `Object[]` value is expected.
>
> Because Array's covariance is potentially unsafe because arrays are
> mutable, in Scala arrays are invariant. That means that
> `Array[TreeNode]` is no subtype of `Array[AnyRef]`.
>
> HTH
> Johannes
>