Re: Constructor
On Fri, 26 Sep 2008, Mark Space wrote:
Eric l wrote:
- Make all the constructors private and use factory methods,
with the drawback that the class can't be extended. (This
approach avoids the quibble mentioned above, assuming the
registration occurs in the factory rather than in the
constructor.)
This is what I would do. Bloch even talks about factory methods in Item 1 of
Effective Java. I think there aren't enough folks using factory methods if
he puts it as #1. They can be clumsy to work with, and seem un-OO, but
factory methods are a great pattern and should be used more (when
appropriate, of course).
I wonder if providing an abstract class to base future classes on would work,
if extension is important. Hmm, but it would be possible to use the class
outside of the factory method. Documentation might work here... so might
relentlessly testing each object passed in to make sure it was registered. A
clever design might even eliminate most of the overhead by only checking on a
few critical methods.
I guess you could call your base class the "implementation" class, then use a
restricted wrapper to get the real object to work on.
public abstract class Implementation {}
final public class Wrapper {
Wrapper( Implementation i ) {} // package private
}
public class Factory {
private Factory() {}
public static Wrapper getNew( Implementation i ) {
Wrapper w = new Wrapper( i );
Registry.register( w ); // not shown
return w;
}
}
This more or less what i was thinking - convert inheritance to
composition, and do the registration in the wrapper after the
implementation is constructed.
I was thinking of having a regular constructor on the wrapper, and doing
it there, but a factory method would be just as good.
Although if you're going to have a factory method, you might as well just
do the registration there:
public static Implementation create() {
Implementation impl = somehowCreateImplementation() ;
Registry.register(impl) ;
return impl ;
}
Although the context in which you could use this is a little different to
what you were describing.
tom
--
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