Comments on: The Filter Pattern: Java Conditional Abstraction With Iterables

By: Fluminis

Fluminis — Thu, 04 Mar 2010 16:06:17 +0000

Yeaaah !, great job, thanks for sharing.

I change your class to filter a tree…
Not so easy but your work help me a lot, thanks again.

By: Erik R.

Erik R. — Sun, 04 Oct 2009 19:55:22 +0000

Yes, that call is valid. The code I have here does compile, after fixing the one error that Phil pointed out above: The iterable parameter on line 11 must be marked final.

Once I fixed that, I just compiled the class fine.

By: Abhijeet

Abhijeet — Sun, 04 Oct 2009 19:44:07 +0000

Thanks for the informative post.

But I can’t get the base class of your code to compile.
Your abstract base class Filter contains a private class, the FilterIterator. This FilterIterator has a toNext method, which internally calls the abstract method passes. This is where my code fails to compile.
“The method passes(T) in the type Filter is not applicable for the arguments (T)”
Is it even valid for an abstract method to be invoked this way?
Or have I got my generics all wrong?

By: Rx

Rx — Mon, 06 Jul 2009 20:02:55 +0000

Thanks Erik – the Filter is a very clever and indeed a VERY powerful pattern. Used it to construct complex filters (composite AND/OR, inverters) on different business objects – very handy and powerful especially with code flexibility and reusability in mind.

to Hardwareguy: you really think so? Well, THAT is why youre the HARDWARE guy.

By: Erik R.

Erik R. — Fri, 15 May 2009 15:16:09 +0000

HG, I guess the two main reasons would be clarity and code reuse. But I suppose one man’s clarity is another man’s obfuscation.

By: Hardwareguy

Hardwareguy — Thu, 14 May 2009 19:34:44 +0000

Should I be the only one to bring up the elephant in the room? The code at the top we’re trying to get rid of is smaller, faster, less error prone, easier to test, more easily readable…I could go on. Why would you want to implement the filter pattern except as a programming exercise?

By: Phil Young

Phil Young — Thu, 04 Dec 2008 18:54:07 +0000

Erik, making the parameter final got past the compilation error. The Filter is a very useful concept. Thanks, Phil

By: Erik R.

Erik R. — Thu, 04 Dec 2008 02:50:21 +0000

No, Phil, I had not seen that. But I’m not sure I actually compiled this code either. It’s meant as an example only. You probably just need to make the parameter “final”, like the compiler says. Maybe I left that out for formatting (so it wouldn’t wrap) reasons. I can’t remember.

The concept is still sound. I use it every day.

By: Phil Young

Phil Young — Wed, 03 Dec 2008 20:33:58 +0000

Erik,

I attempted to compile your Filter class with JRE 6.0_07 but I got an error here:
public Iterable filter(Iterable iterable) {
return new Iterable() {
public Iterator iterator() {
//** next line: “cannot refer to a non-final variable iterator inside an inner class defined in a different method.
return filter(iterable.iterator());
}
};
}

Have you seen this error?
Thanks, Phil Young, Xerox Corp.

By: Heather

Heather — Fri, 18 Jan 2008 23:47:33 +0000

I didn’t know it was possible to feel intelligent and mind blowingly dumb at the same time………