The "Closure" in commons collections is not named well: for
non-functional programmers it will induce a "what's that?", and for
functional programmers it will confuse expectations.
From http://en.wikipedia.org/wiki/Closure_(computer_science):
A closure combines the code of a function with a special lexical
environment bound to that function (scope).
Java cannot pass functions, so the only way this can be done is with an
(inner) class, as follows (also from wikipedia):
class CalculationWindow extends JFrame {
private JButton btnSave;
...
public final calculateInSeparateThread(final URI uri) {
// The expression "new Runnable() { ... }" is an anonymous class.
Runnable runner = new Runnable() {
void run() {
// It can access final local variables:
calculate(uri);
// It can access private fields of the enclosing class:
btnSave.setEnabled(true);
}
};
new Thread(runner).start();
}
}
Note how the Runnable accesses the btnSave variable in the function
scope. This "special lexical environment" is NOT the same as passing a
parameter through a constructor OR execute method. A Closure should NOT
take a parameter for execute. AFAIK, a true Closure can only be
implemented in Java by means of a non-static inner class.
See also http://www.blog.dannynet.net/archives/39, and
http://codefu.taichizus.net/2006/02/19/closures-in-java/ for (IMHO)
understanding and mis-understanding of closures in java.
So, what to do?
I would propose an interface called Processor. It is more intuitive and
has many "real world" examples that can anchor the term so that it makes
sense to the average (and functional!) programmer.
For example, when applying for a passport, some documentation needs to
be filled out, and then it will go through a process to get you a
passport. You hand (or send) your forms to a clerk (Processor), and
that's it. The Processor does not reply - the context that is passed in
your form (your details) allows a message to be sent back at a later date.
My proposed class would be
public interface Processor<T> extends Closure<T>{}
with the appropriate documentation.
Closure would be deprecated with an appropriate explanation. (I would
advocate removing it by 2008, or when the Java "standard" is Java 5
instead of Java 2)
Cheers
Stephen Kestle
PS. Apologies if this topic has been covered before - I didn't see a
search on the mailing list archives...
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