<Swing Dev> 6179357: Generics: JList: ListCellRenderer & prototypeCellValue

[Florian Brunner]

Hi Alexander,

Alexander Potochkin schrieb:
> Hello Florian
>
> Thank you for the constructive examples!
>
You're welcome! :-)
> I looked at the usage of prototypeCellValue property,
> following the JList.updateFixedCellSize() method
>
Yes, this was the reason for this whole discussion: the list entries and 
the prototypeCellValue use the same ListCellRenderer.
If both are of type E, both can use a ListCellRenderer<? super E>

> I found this code in the 
> DefaultListCellRenderer.getListCellRendererComponent():
>
>
>     if (value instanceof Icon) {
>         setIcon((Icon)value);
>         setText("");
>     }
>     else {
>         setIcon(null);
>         setText((value == null) ? "" : value.toString());
>     }
>
> It means that value always can be an icon,
> so we need to do something about that

I already had a short look at DefaultListCellRenderer. I think it has to 
be specified something like:

DefaultListCellRenderer implements ListCellRenderer<Object>

since it works for all Objects (toString() ) and Object is a superclass 
of Icon. Like this it also fits to ListCellRenderer<? super E> for any 
given E and thus should be possible to use it as the default 
cellRenderer of JList (which it probably already is, though I can't 
check it right now. Somewhere in the UI class?)

I didn't plan to include DefaultListCellRenderer in the first patch, but 
can do so, if you prefer.
>
> I also think that shouldn't generify prototypeCellValue,
> since it doesn't give as any visible advantage,
This would be option 1) then, because if prototypeCellValue is of type 
Object, the ListCellRenderer has to be as well. The value of having a 
generified prototypeCellValue is that we can use a generified 
ListCellRenderer!

I think it would be a pity if we don't provide a generic ListCellRenderer.
> moreover some programmers may use an object of a special type
> that toString() method returns something meaningful for 
> prototypeCellValue
>
This is still possible with option 3), eg. with JList<Object> or "raw" 
JList.

-Florian

> Thanks
> alexp
>
>> Here is a second sample.
>>
>> It's not really a  real-world sample, but should be useful to show 
>> that also JList of other "value" types than String, such as Integer, 
>> Long and Short, can profit from generics.
>>
>> It also shows why JList should specify
>> ListCellRenderer<? super E> cellRenderer
>>
>> rather than
>>
>> ListCellRenderer<E> cellRenderer
>>
>> (Here: a common Number-cell renderer is used.)
>>
>> -Florian
>>
>> Am Dienstag, 3. März 2009 schrieb Alexander Potochkin:
>>> Hello Tom
>>>
>>> It's nice to see you here
>>>
>>>>> Could you please provide a complete example of a JList
>>>>> with a custom ListCellRenderer that proves that renderer should be
>>>>> generified
>>>> I bet if you used NetBeans to find implementations of ListCellRenderer
>>>> even within the JDK most useful implementations would cast the value
>>>> argument.
>>> anyway, it is always better to have a fixed set of examples to discuss
>>>
>>>> (I prefer option 3, btw.)
>>> Thanks
>>> alexp
>>>
>>>> Tom Hawtin
>>
>>
>