Closure is an inaccurate name
-----------------------------
Key: COLLECTIONS-233
URL: http://issues.apache.org/jira/browse/COLLECTIONS-233
Project: Commons Collections
Issue Type: Improvement
Components: Collection
Affects Versions: Generics
Reporter: Stephen Kestle
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. 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 explanation.
However, it may be acceptable with the new version just to do a rename.
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