Re: passing a Factory to a method to create a generic instance

Tom Anderson wrote:
On Sat, 10 May 2008, thufir wrote:

On Sat, 10 May 2008 01:19:13 +0100, Tom Anderson wrote:

Before i go on, i should say that in the classical application of the
Factory pattern, yes, you would make something more specific than an
Object, because the different factories would be making different
versions of something, or the same thing in different ways. Like you
might define a WidgetFactory, then have concrete factories that make
Nut, Bolt, Screw, etc objects, all of which are subtypes of Widget.

Does there have to be a Nut factory, or can I just use the Widget factory?

I'm not seeing the advantage of WidgetFactory, because I don't seem able
to use it.

WidgetFactory is abstract. NutFactory etc are concrete implementations of it. Sorry if i haven't explained this clearly.

abstract class Widget {
}

class Nut extends Widget {
}

class Bolt extends Widget {
}

abstract class WidgetFactory {
abstract Widget make() ;
}

class NutFactory extends WidgetFactory {
Widget make() {
return new Nut() ;
}
}

class BoltFactory extends WidgetFactory {
Widget make() {
return new Bolt() ;
}
}

// a usage example
class CratePacker
{
Crate pack(int number, WidgetFactory fac) {
Crate c = new Crate() ;
for (int i = 0 ; i < number ; ++i)
c.add(fac.make()) ;
return c ;
}
}

The point is to be able to pack crates of widgets with one method, which can be parameterised with a factory which defines the kind of widget.

Closures? We don't need no steenkin' closures!

In this simple example, Wdiget should be an interface rather than a class, but that in no wise detracts from the main point of the example.

--
Lew
.

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