Type problems with native Java classes.

Question

Hello,

I have the following problem, maybe someone can help me (or explain, where my mistake is). Given are a trait and a class:

trait TextAttr[T <: {def setText(s:String); def getText : String}] {
  val obj : T
  def txt_= (s:String) = obj.setText(s)
  def txt = obj.getText
}

class ScalaText {
  private var t = ""
  def setText(s:String) = t = s
  def getText = t
}

Now I create a new class using both of them:

class ScalaTextUser extends TextAttr[ScalaText] {
  override val obj = new ScalaText
}

That's okay. But if I want to create something like that with the class org.eclipse.swt.widgets.Text (or any other pure Java class) I get an error. Here the code:

class SwtTextUser(parent:Composite) extends TextAttr[Text] {
  override val obj = new Text(parent, 0)
}

And this is the error:

type arguments [org.eclipse.swt.widgets.Text] do not conform to trait TextAttr's type parameter bounds [T <: AnyRef{def setText(String): Unit; def getText: String}]

Has anybody an idea?

Thanks, Chris.

Answer 1

Try changing the trait declaration to the following:

trait TextAttr[T <: {def setText(s: String); def getText(): String}] {

Scala has a subtly-different treatment for methods with and without parentheses. When in doubt, add the parens and let the compiler sort things out.