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From "Stephen Kestle (JIRA)" <j...@apache.org>
Subject [jira] Created: (COLLECTIONS-290) AllPredicate should be able to cast objects
Date Wed, 26 Mar 2008 00:43:24 GMT
AllPredicate should be able to cast objects
-------------------------------------------

                 Key: COLLECTIONS-290
                 URL: https://issues.apache.org/jira/browse/COLLECTIONS-290
             Project: Commons Collections
          Issue Type: Improvement
          Components: Functor
    Affects Versions: Generics
            Reporter: Stephen Kestle
            Assignee: Stephen Kestle
             Fix For: Generics


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 for.

I propose that InstanceOfPredicate has a signature like:
{code}public final class InstanceofPredicate<T,CT> implements Predicate<T>, Serializable
...
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 safety.

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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