[OpenJDK 2D-Dev] <AWT Dev> What to override and what now

[Roman Kennke]

Hi Artem,

> > While working a bit on cacio, we just found some new nice addition to the Toolkit code, like this one:
> >
> >      public boolean areExtraMouseButtonsEnabled() throws HeadlessException {
> >          GraphicsEnvironment.checkHeadless();
> >
> >          return Toolkit.getDefaultToolkit().areExtraMouseButtonsEnabled();
> >      }
> >
> > Of course, this method is meant to be overridden, so it will end up doing nothing, but things like this may be very difficult to debug at times, and in any case, is definitely are not friendly code. Is there any reason for this smartness?
> 
> I completely agree. This code seems to be never called, otherwise it 
> would fail into an infinite recursion, it shouldn't be any problems with 
> it returning "false".

Why not make it throw an exception? This way it would be very easy to
find out what is wrong. Returning null, false, -1 etc might make your
code blow up much later and it takes some debugging to figure out what
is wrong.

> > Why not simply provide a default implementation or just throw an exception or simply make the method abstract? Or maybe I'm missing something?
> 
> Making it abstract is not a good idea: I know some rare cases when the 
> Toolkit class is extended (e.g. for testing purposes), and adding a new 
> abstract method will break such code at compile time.

Yeah agreed. We are doing this massively in Cacio as you can imagine,
and for us would be nicer to have a compile time error so we know what
to fix. But I see that java.awt.Toolkit is public API and cannot be
changed in a non-compatibel way...

Cheers, Roman