General talk about software patterns

[James Siddle <james.siddle AT uk.ibm.com>]

Hi,

I was wondering if anyone has examples
of successful combinations of the Command, Composite, and Visitor patterns
in Java. I'm attempting to introduce Visitor into a Composite & Command
implementation (as described in Chapter 6 of POSA5), but have run into
a problem. 

In my current design, a "Command"
interface is implemented by concrete command classes, and a CompositeCommand
class implements Command in order to aggregates other commands. This means
I can hierarchically compose commands. 

What I'm struggling to know is how to
introduce a "Visitable" interface onto the Command classes, without
tying the Command interface to the Visitor interface. I've narrowed my
choices to three options, so any guidance on which one to go for would
be appreciated :-)

The options are:
1) Add 'accept' onto the Command interface
-> means that Command interface will indirectly have knowledge of concrete
commands, by knowing about the Visitor interface.
2) Have a separate Visitable interface
-> means that my CompositeCommand class will have to use runtime type
information when passing on accept calls to aggregated commands, which
may not implement Visitable
3) Introduce a VisitableCommand interface,
and change concrete command classes to implement it -> this option just
isn't feasible in my current configuration partly because it would involve
changing code which it just isn't possible to change

Any thoughts appreciated,

Thanks,
Jim


James Siddle
IBM WebSphere ESB Foundation Technologies
telephone: +44 1962 81 6055
email: james.siddle AT uk.ibm.com
personal webpage: www.jamessiddle.net

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