Problem with Java Generics and Wildcards in Scala

Hi John,

thanks for your feedback. But it doesn't help. When I change the method
signature to

override def getListCellRendererComponent(list: JList[Object],
value: Object, index: Int, isSelected: Boolean,
cellHasFocus: Boolean): Component = { ... }

then I get this error:

"method getListCellRendererComponent overrides nothing"

Another ideas?

Bye, Stefan

Am 18.08.2011 00:28, schrieb John L. Cheng:
> I don't have Java 8 installed, but since DefaultListCellRenderer
> implements ListCellRenderer instead of ListCellRenderer<?>,
> does that mean you need to parameterise with "Object" as well?
>
> class ScalaListCellRenderer extends DefaultListCellRenderer {
> override def getListCellRendererComponent(
> list: JList[Object], value: Object, index: Int, isSelected:
> Boolean,
> cellHasFocus: Boolean): Component = {
> val component = super.getListCellRendererComponent(
> list, value, index, isSelected, cellHasFocus)
> // ... do some component customization here ...
> component
> }
> }
>
> Note JList[Object} in the method signature
>
> On Aug 17, 1:30 pm, Stefan Saring wrote:
>> Hi Scala users,
>>
>> I'm not able to solve a problem with Java Generics and Wildcards in
>> Scala, maybe some experts can help me here. Or maybe it's a bug - I
>> don't know...
>>
>> I'm trying to implement an extension of the DefaultListCellRenderer
>> (Swing), which is needed for a List in my Swing application. There were
>> no problems with Java 6.
>>
>> I needed to change to Java 7, which has introduced a minor change in the
>> API of the interface ListCellRenderer and the class
>> DefaultListCellRenderer - Java Generics were added.
>>
>> Here's the API-excerpt of DefaultListCellRenderer.java:
>>
>> public class DefaultListCellRenderer extends JLabel
>> implements ListCellRenderer, Serializable {
>>
>> public Component getListCellRendererComponent(JList<?> list,
>> Object value, int index, boolean isSelected,
>> boolean cellHasFocus) {...}
>> ...
>>
>> }
>>
>> And here's the API-excerpt of ListCellRenderer.java:
>>
>> public interface ListCellRenderer {
>>
>> Component getListCellRendererComponent(JList<? extends E> list,
>> E value, int index, boolean isSelected, boolean cellHasFocus);
>> ...
>>
>> }
>>
>> And here is my extension of class DefaultListCellRenderer, which can't
>> be compiled (tested with Scala 2.8.1 and 2.9.0.1):
>>
>> import java.awt.Component
>> import javax.swing.{DefaultListCellRenderer, JList}
>>
>> class ScalaListCellRenderer extends DefaultListCellRenderer {
>>
>> override def getListCellRendererComponent(
>> list: JList[_ <: Object], value: Object, index: Int,
>> isSelected: Boolean, cellHasFocus: Boolean): Component = {
>>
>> val component = super.getListCellRendererComponent(
>> list, value, index, isSelected, cellHasFocus)
>> // ... do some component customization here ...
>> component
>> }
>>
>> }
>>
>> The compiler error is:
>>
>> ambiguous reference to overloaded definition, both method
>> getListCellRendererComponent in class DefaultListCellRenderer of type
>> (x$1: javax.swing.JList[_], x$2: Any, x$3: Int, x$4: Boolean, x$5:
>> Boolean)java.awt.Component and method getListCellRendererComponent in
>> trait ListCellRenderer of type (x$1: javax.swing.JList[_ <:
>> java.lang.Object], x$2: java.lang.Object, x$3: Int, x$4: Boolean, x$5:
>> Boolean)java.awt.Component match argument types
>> (javax.swing.JList[_$1],java.lang.Object,Int,Boolean,Boolean)
>>
>> I've also tried to change the Generics wildcard to JList[_]:
>>
>> class ScalaListCellRenderer extends DefaultListCellRenderer {
>>
>> override def getListCellRendererComponent(
>> list: JList[_], value: Object, index: Int, isSelected: Boolean,
>> cellHasFocus: Boolean): Component = {
>>
>> val component = super.getListCellRendererComponent(
>> list, value, index, isSelected, cellHasFocus)
>> // ... do some component customization here ...
>> component
>> }
>>
>> }
>>
>> But then I'm getting this compiler error:
>>
>> class ScalaListCellRenderer needs to be abstract, since method
>> getListCellRendererComponent in trait ListCellRenderer of type (x$1:
>> javax.swing.JList[_ <: java.lang.Object], x$2: java.lang.Object, x$3:
>> Int, x$4: Boolean, x$5: Boolean)java.awt.Component is not defined
>>
>> I can't find a solution for this problem. Does somebody has an idea or
>> is it not possible to solve the problem?
>>
>> Thanks in advance,
>> Stefan
>

Yes, for me it seems that Scala is not able to handle this Java<->Scala
Generics conflict. But I'm not sure whether this is a Scala problem /
bug or not...

Am 18.08.2011 18:18, schrieb John Cheng:
> Its weird that these posts are not showing up in the user group. I did
> download J7SE and test out the scenario and started seeing the same
> problem you did.
>
> Scala is recognizing DefaultListCellRenderer and ListCellRenderer as
> having separate method signatures.
>
> On Thu, Aug 18, 2011 at 8:32 AM, Stefan Saring wrote:
>> Hi John,
>>
>> thanks for your feedback. But it doesn't help. When I change the method
>> signature to
>>
>> override def getListCellRendererComponent(list: JList[Object],
>> value: Object, index: Int, isSelected: Boolean,
>> cellHasFocus: Boolean): Component = { ... }
>>
>> then I get this error:
>>
>> "method getListCellRendererComponent overrides nothing"
>>
>> Another ideas?
>>
>> Bye, Stefan
>>
>>
>> Am 18.08.2011 00:28, schrieb John L. Cheng:
>>> I don't have Java 8 installed, but since DefaultListCellRenderer
>>> implements ListCellRenderer instead of ListCellRenderer<?>,
>>> does that mean you need to parameterise with "Object" as well?
>>>
>>> class ScalaListCellRenderer extends DefaultListCellRenderer {
>>> override def getListCellRendererComponent(
>>> list: JList[Object], value: Object, index: Int, isSelected:
>>> Boolean,
>>> cellHasFocus: Boolean): Component = {
>>> val component = super.getListCellRendererComponent(
>>> list, value, index, isSelected, cellHasFocus)
>>> // ... do some component customization here ...
>>> component
>>> }
>>> }
>>>
>>> Note JList[Object} in the method signature
>>>
>>> On Aug 17, 1:30 pm, Stefan Saring wrote:
>>>> Hi Scala users,
>>>>
>>>> I'm not able to solve a problem with Java Generics and Wildcards in
>>>> Scala, maybe some experts can help me here. Or maybe it's a bug - I
>>>> don't know...
>>>>
>>>> I'm trying to implement an extension of the DefaultListCellRenderer
>>>> (Swing), which is needed for a List in my Swing application. There were
>>>> no problems with Java 6.
>>>>
>>>> I needed to change to Java 7, which has introduced a minor change in the
>>>> API of the interface ListCellRenderer and the class
>>>> DefaultListCellRenderer - Java Generics were added.
>>>>
>>>> Here's the API-excerpt of DefaultListCellRenderer.java:
>>>>
>>>> public class DefaultListCellRenderer extends JLabel
>>>> implements ListCellRenderer, Serializable {
>>>>
>>>> public Component getListCellRendererComponent(JList<?> list,
>>>> Object value, int index, boolean isSelected,
>>>> boolean cellHasFocus) {...}
>>>> ...
>>>>
>>>> }
>>>>
>>>> And here's the API-excerpt of ListCellRenderer.java:
>>>>
>>>> public interface ListCellRenderer {
>>>>
>>>> Component getListCellRendererComponent(JList<? extends E> list,
>>>> E value, int index, boolean isSelected, boolean cellHasFocus);
>>>> ...
>>>>
>>>> }
>>>>
>>>> And here is my extension of class DefaultListCellRenderer, which can't
>>>> be compiled (tested with Scala 2.8.1 and 2.9.0.1):
>>>>
>>>> import java.awt.Component
>>>> import javax.swing.{DefaultListCellRenderer, JList}
>>>>
>>>> class ScalaListCellRenderer extends DefaultListCellRenderer {
>>>>
>>>> override def getListCellRendererComponent(
>>>> list: JList[_ <: Object], value: Object, index: Int,
>>>> isSelected: Boolean, cellHasFocus: Boolean): Component = {
>>>>
>>>> val component = super.getListCellRendererComponent(
>>>> list, value, index, isSelected, cellHasFocus)
>>>> // ... do some component customization here ...
>>>> component
>>>> }
>>>>
>>>> }
>>>>
>>>> The compiler error is:
>>>>
>>>> ambiguous reference to overloaded definition, both method
>>>> getListCellRendererComponent in class DefaultListCellRenderer of type
>>>> (x$1: javax.swing.JList[_], x$2: Any, x$3: Int, x$4: Boolean, x$5:
>>>> Boolean)java.awt.Component and method getListCellRendererComponent in
>>>> trait ListCellRenderer of type (x$1: javax.swing.JList[_ <:
>>>> java.lang.Object], x$2: java.lang.Object, x$3: Int, x$4: Boolean, x$5:
>>>> Boolean)java.awt.Component match argument types
>>>> (javax.swing.JList[_$1],java.lang.Object,Int,Boolean,Boolean)
>>>>
>>>> I've also tried to change the Generics wildcard to JList[_]:
>>>>
>>>> class ScalaListCellRenderer extends DefaultListCellRenderer {
>>>>
>>>> override def getListCellRendererComponent(
>>>> list: JList[_], value: Object, index: Int, isSelected: Boolean,
>>>> cellHasFocus: Boolean): Component = {
>>>>
>>>> val component = super.getListCellRendererComponent(
>>>> list, value, index, isSelected, cellHasFocus)
>>>> // ... do some component customization here ...
>>>> component
>>>> }
>>>>
>>>> }
>>>>
>>>> But then I'm getting this compiler error:
>>>>
>>>> class ScalaListCellRenderer needs to be abstract, since method
>>>> getListCellRendererComponent in trait ListCellRenderer of type (x$1:
>>>> javax.swing.JList[_ <: java.lang.Object], x$2: java.lang.Object, x$3:
>>>> Int, x$4: Boolean, x$5: Boolean)java.awt.Component is not defined
>>>>
>>>> I can't find a solution for this problem. Does somebody has an idea or
>>>> is it not possible to solve the problem?
>>>>
>>>> Thanks in advance,
>>>> Stefan
>>>
>>
>>
>
>
>

Does this mean that it's not possible to solve the problem at the
moment? Or is there any workaround without patching the JDK 7 ?

Bye, Stefan

Am 18.08.2011 18:35, schrieb Paul Phillips:
> https://issues.scala-lang.org/browse/SI-3634
> https://github.com/scala/scala/tree/openjdk
>
> Remedies are presently under discussion on scala-internals.
>

OK, but how should it be possible to compile against Java 7 only?
I'm not able to extend the class DefaultListCellRenderer of Java 7 (as
shown in the first post of this topic).

Am 18.08.2011 19:02, schrieb Paul Phillips:
> On 8/18/11 9:56 AM, Stefan Saring wrote:
>> Does this mean that it's not possible to solve the problem at the
>> moment? Or is there any workaround without patching the JDK 7 ?
>
> It means it is not at present possible to compile against jdk6 and jdk7
> with the same scala source base if you use one of the classes which
> acquired type parameters.
>

Finally I was able to solve the problem with a simple workaround. Here's
the solution for extending the DefaultListCellRenderer (Task is my
domain class):

object TaskCellRenderer extends ListCellRenderer[Task] {

val peerRenderer: ListCellRenderer[Task] = (new
DefaultListCellRenderer).asInstanceOf[ListCellRenderer[Task]]

override def getListCellRendererComponent (
list: JList[_ <: Task], task: Task, index: Int,
isSelected: Boolean, cellHasFocus: Boolean): Component = {

val component = peerRenderer.getListCellRendererComponent(
list, task, index, isSelected, cellHasFocus)
.asInstanceOf[JComponent]

// ... do some component customization here ...

component
}
}

Instead of extending the DefaultListCellRenderer directly I implement
the ListCellRenderer interface. This has a casted
DefaultListCellRenderer as a peer which is beeing used as a delegate in
the implementation of getListCellRendererComponent(...).
This works without problems with the unmodified Java 7 and the
unmodified Scala 2.9.0.1.

Thanks to all for the help!

Bye, Stefan

Am 18.08.2011 19:39, schrieb Paul Phillips:
> On 8/18/11 10:08 AM, Stefan Saring wrote:
>> OK, but how should it be possible to compile against Java 7 only?
>> I'm not able to extend the class DefaultListCellRenderer of Java 7 (as
>> shown in the first post of this topic).
>
> That's why I sent you the links. Scala can't build against both of them
> simultaneously any better than you can, which means you need a build of
> scala against java7.
>
> https://github.com/scala/scala/tree/openjdk
>