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From "Stephen Kestle (JIRA)" <>
Subject [jira] Commented: (COLLECTIONS-290) AllPredicate should be able to cast objects
Date Wed, 26 Mar 2008 00:45:26 GMT


Stephen Kestle commented on COLLECTIONS-290:

AllPredicate and InstanceOfPredicate would both extend the same abstract base class (and an
interface) for the casting convinience methods.

> AllPredicate should be able to cast objects
> -------------------------------------------
>                 Key: COLLECTIONS-290
>                 URL:
>             Project: Commons Collections
>          Issue Type: Improvement
>          Components: Functor
>    Affects Versions: Generics
>            Reporter: Stephen Kestle
>            Assignee: Stephen Kestle
>             Fix For: Generics
>   Original Estimate: 4h
>  Remaining Estimate: 4h
> When using the generified AllPredicate class (in it's current form), it requires instantiation
with all predicates having generic supertypes of the resultant combination:
> {code}public static <T> Predicate<T> getInstance(Predicate<? super T>
... predicates){code}
> However, if one of the predicates was an InstanceOfPredicate, it could mean that every
other predicate is able to be a super of the type that the instanceOfPredicate returns true
> I propose that InstanceOfPredicate has a signature like:
> {code}public final class InstanceofPredicate<T,CT> implements Predicate<T>,
> ...
> public static <T,CT> InstanceofPredicate<T,CT> getInstance(Class<CT>
type) {code}
> Where CT is the Type of the class that will be matched by the predicate.
> AllPredicate should have constructors like:
> {code}public static <T,CT> AllPredicate<T,CT> getInstance(InstanceOfPredicate<?,CT>,
Predicate<? super CT> ... predicates)
> public static <T,CT> AllPredicate<T,CT> getInstance(Class<CT>, Predicate<?
super CT> ... predicates){code}
> This would allow far nicer constructs of anonymous or simple predicates, and better type
> There would also be an additional method on AllPredicate:
> {code}public CT cast(Object matched){code}
> That could be used as follows:
> {code}Predicate<Integer> predicate = AllPredicate.instanceOf(Integer.class, ...);
> Object o= ...;
> if (predicate.evaluate(o)){
>     Integer i = predicate.cast(o);
>      //perform action
> }{code}
> Alternatively there could be a different method that attempts to coerce the type:
> {code}public CT transform(T t){
>     if (evaluate(t)){
>         return clazz.cast(t);  //clazz is of type Class<CT>
>     }
>     return null;
> }{code}
> This could be used as follows:
> {code}Predicate<Integer> predicate = AllPredicate.instanceOf(Integer.class, ...);
> Object o= ...;
> Integer i = predicate.transform(o)
> if (i != null){
>     //perform action
> }{code}
> I prefer the cast() method; however, it would not re-evaluate the object, so would need
documentation to be used correctly.  The second method would allow it to be used as a transformer,
but would substitute boolean checking to null checking.
> Perhaps both  are warranted.  (The reason for the convenience methods are to not generate
type-coercion warnings).

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