On 09/18/2010 10:36 AM, Simon Brooke wrote:
I'm looking for the most idiomatic and elegant means of iterating
over a
filtered subset of a collection. Here's the basic structure of piece of
code I'm looking at, which must be fairly common:
Vector<Widget> widgets = doSomethingToGetWidgets();
for (Widget widget : widgets) {
if (widget instanceof ActionWidget) {
doSomethingWithActionWidget( (ActionWidget) widget);
}
}
(obviously, ActionWidget is a subclass of Widget)
What I'd like to do would be something like
Vector<Widget> widgets = doSomethingToGetWidgets();
Wha...??? Vector? Really? Come on! You're just yanking our chain, right?
No, really, 'fess up. You're pulling our leg, aren't you?
Aren't you?
for (ActionWidget widget : widgets
where (widget instanceof ActionWidget)) {
doSomethingWithActionWidget( (ActionWidget) widget);
}
I can't find anything in the Java 5 collections documentation which
offers type filtering functionality; am I missing something?
Yeah, that what you did there is an antipattern. Use proper object
orientation and the problem magically melts away.
Instead of 'doSomethingWith( Foo foo )' implement 'Foo.doSomething()'.
Then you get type-based execution as a proper concomitant to
polymorphism. That/s the whole freaking *POINT* of object-orientation,
for Pete's sake!
for( Widget widget : somehowGetWidgets() )
{
widget.doSomething();
}
Then 'ActionWidget' subclass instances will do the
'ActionWidget#doSomething()' override and 'PassionWidget' subclass
instances will do the 'PassionWidget#doSomething()' override, each
doing the right thing for its own type automagically without silly
'instanceof' tests.
If you really need your iteration to happen only over 'ActionWidget'
instances there really isn't anything inbuilt in Java to do what you
asked for without an explicit 'if ( widget instanceof ActionWidget )'
test, but the very presence of that test is a red flag that you got
your object model wrong.
If you don't have a 'Collection <ActionWidget>' in the first place
your problem is upstream.
I don't entirely agree. Suppose you have a collection of widgets of
seems quite reasonable to me. Yes, you could make all Widgets implement