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From "Brian Egge (JIRA)" <>
Subject [jira] Commented: (COLLECTIONS-233) Closure is an inaccurate name
Date Tue, 30 Oct 2007 10:45:50 GMT


Brian Egge commented on COLLECTIONS-233:

In some future version of Java, we may get Closures.  It hasn't been voted into 7.0 yet. 
I think the Closures offered by the Collections is currently the closest thing you can get
in Java.  True, it not a real closure like what you get in Smalltalk or Ruby.  Having learned
to use Closures in Ruby, I found it easy to pick up in the Collections framework.  Having
another name would make the process more complex.  If/when Java comes up with real closures,
we could depreciate the Closures interface, and give a good example of how to convert it to
a real one.

Introducing the Processor interface, would just create additional refactoring, if the end
goal is to use a built in Java mechanism instead.  

I vote "won't fix" on this issue - at least not for now.  Maybe if the 1.4 branch was in depreciated
status, and there was a clear idea of if Closures are going to be introduced into the Java
languages, and if they are, what exactly the usage is going to be.  (As I understand there
are two competing ways of implementing closures).

> Closure is an inaccurate name
> -----------------------------
>                 Key: COLLECTIONS-233
>                 URL:
>             Project: Commons Collections
>          Issue Type: Improvement
>          Components: Collection
>            Reporter: Stephen Kestle
>             Fix For: Generics
> 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 
> 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.  It is not actually possible to
have a "Closure" object, as that breaks the lexical environment.
> 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 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. 
> For backwards compatibility the interface would be  
>         public interface Processor<T> extends Closure<T>{} 
> with the appropriate documentation.   Closure would be deprecated with an appropriate
> However, it may be acceptable with the new version just to do a rename.

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