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

"Mike Schilling" <>
Sat, 18 Sep 2010 18:53:31 -0700
"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,

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'

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

And just for (a sufficiently warped version of) fun, here's a class that
enables iterating over a type-filtered collection:

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

public class FilteredCollection<T, C> implements Iterable<C>
    private Iterable<T> collection;
    private Class<C> filter;

    private FilteredCollection(Iterable<T> collection, Class<C> filter)
        this.collection = collection;
        this.filter = filter;

    public static <T, C> Iterable<C> getFilteredCollection(Iterable<T>
collection, Class<C> filter)
        return new FilteredCollection<T, C>(collection, filter);

    public Iterator<C> iterator()
        return new FilteredIterator();

    private class FilteredIterator implements Iterator<C>
        private Iterator<T> iterator;
        private C nextObject;

        private FilteredIterator()
            iterator = collection.iterator();

        public boolean hasNext()
            return nextObject != null;

        public C next()
            if (nextObject == null)
                throw new NoSuchElementException();
            C retval = nextObject;
            return retval;

        public void remove()
            throw new UnsupportedOperationException();

        protected void fill()
            while (iterator.hasNext())
                T next =;
                if (filter.isInstance(next))
                    nextObject = filter.cast(next);
            nextObject = null;

    public static void main(String[] args)
        List<?> objects = Arrays.asList(1, 2.2, "a", null, false, "b");
        for (String s : getFilteredCollection(objects, String.class))

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