Re: A filtered iteration over a collection: current idiom?

Lew <>
Sat, 18 Sep 2010 20:38:13 -0400
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() )

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.


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