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

Daniel Pitts <>
Sun, 19 Sep 2010 18:38:23 -0700
On 9/18/2010 7:32 PM, Arne Vajh?j wrote:

On 18-09-2010 21:53, Mike Schilling wrote:

"Lew" <> wrote in message

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.

I don't entirely agree. Suppose you have a collection of widgets of
different types, some of which require explicit actions when disposed,
and some of which do not.

for (Widget widget : widgetCollection)
if (widget instanceof Disposable)

seems quite reasonable to me. Yes, you could make all Widgets implement
a no-op dispose() method, but that becomes more onerous as the number of
such optional features increases, unless all Widgets derive from a
common base class.

Stuff like that is frequently seen in real world Java.

I will claim that is almost all cases, then the object
model could have been designed better to avoid the need.

But in the real world we are usually stuck with the
object model once created forever.

I almost agree.
Young projects change easily.
Older projects (6 mo after first launch maybe?) change with slight effort.
Older yet projects (1 year after first launch?) change with modest effort.

Refactoring larger/older projects takes effort, especially if the
architecture itself needs to be rethought, however it is not impossible.
  The difficulty comes from the following dilemma: I have 1 week to
implement a 6 new features. To do it "cleanly" will take 2 weeks, to do
it with an "if instance of" will take 3 days.

My boss was pleased it only took 3 days, so doubled the feature requests
for next week. No time to go back and fix the code.

This dilemma can be mitigated by managing expectations. "I can get a
throw-away prototype of this feature out in 3 days, but it will need to
be re-written after we launch it."

Anyway, type safe filtering Iterable is a useful utility.
Daniel Pitts' Tech Blog: <>

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