Subclassing "Not inteded for subclassing"

Aarti_pl aarti at
Wed Oct 15 00:07:50 PDT 2008

Frank Benoit pisze:
> Simen Haugen schrieb:
>> I see dwt also have the "Not intended for subclassing", but the classes
>> are not marked final.
>> It shouldn't be any problems subclassing as long as you're careful, right?
>> Right now I want to subclass Text and automatically add a dynamic
>> tooltip. The same type of text box should be used many places in my
>> application. I know I can just bind each time, but is it really
>> dangerous to do this kind of subclassing to save typing?
>> Or is there a easier way to solve these kind of things?
> Hehe, i don't know why, but i simply forgot about making them 'final'.
> Perhaps i should do that :)
> Well, some of them are subclassed within SWT, but they shall not be
> subclasses by user code. (Decorations, Scrollable, ..)
> I think, they are not intended for subclassing, because of ...
> 1. So you do not depend on protected/private API and the SWT project is
> free in changing it in a future version.
> 2. So you do not used protected/private API which may be platform
> dependent. There are examples of this.
> 3. So you do not accidentally change any behavior of the widget, which
> might be expected from other classes, like layouts or widget containers.
> I am convinced, the SWT devs are very good designers and they choose
> this restriction for good reason.
> Perhaps you can do this by composition instead of inheritance.
> Do you have code examples of what you want to do?

Frank, please do not make them final! :-O

I use a few subclassed SWT widgets in my code, and have no problem 
with them.

To subclass widget it is enough to override method checkSubclass() with 
empty body:
protected void checkSubclass() {

I think that there should be no problem with subclassing as long as you 
use there only public interface of SWT.

Finally it is sometimes more convenient to keep updating subclassed 
widget (because of possible SWT api changes), than make them as 
aggregated composites and redirect all necessary methods from original 
widget to new widget. That's why I think even SWT doesn't declare them 

Best Regards
Marcin Kuszczak

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