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

Lew <>
Sat, 18 Sep 2010 22:31:32 -0400
Mike Schilling wrote:

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

Brilliant. Bravo.

You show how to put type safety back into what would otherwise be too risky.

Reflective type tricks work best when buried in type-safe utility classes like

Applause also for "when the API don't got it, yer roll yer own as if 'twere in
the API." Very Java-like code, a perfect example of type-oriented programming.

Good API writing and good example of maintainability, too.

Good show.

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

Single-type imports, a best practice.

public class FilteredCollection<T, C> implements Iterable<C>

'Iterable' is the least-most broad type applicable, a best practice.

("Least-most broad" - types should be the broadest applicable but no broader
than strictly necessary. A generalization of "prefer interface types".)

Implementation of a standard API type is a best practice.

'Iterable' is a great workhorse type.

Shouldn't that class definition be
   public class FilteredCollection <T, C extends T> implements Iterable<C>

   private Iterable<T> collection;
   private Class<C> filter;

Classic run-time type token.

   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 )

Just to remind the public of a gotcha - the type parameters of a static method
are not the same as those of the generic class type itself, which are bound to
instances. The 'T' and 'C' in this static method could have equivalently been
'U' and 'F':

   public static <U, F extends U>
     Iterable <F> getFilteredCollection(
       Iterable <U> collection, Class <F> filter )

It's a beautiful thing how this declaration supports type inference for teh

   Iterable <ActionWidget> actionWidgets = getFilteredCollection(
       getAllWidgets(), ActionWidget.class );

where 'getAllWidgets()' returns a collection (rather, iterable) of some
supertype of 'ActionWidget'.

Mike's example uses type inference to make a compact 'for-each' expression.

       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))

Run-time type token is better and safer than 'instanceof' chains.

                   nextObject = filter.cast(next);

Also better than hard-coded casts.

           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))

There's that slick use of type inference.


Thanks, Mike.


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