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From Chris Lambrou <>
Subject Re: [collections] Classes that decorate collections with References?
Date Sat, 25 Dec 2004 00:41:12 GMT
Bastian Helfert wrote:

>"Jakarta Commons Developers List" <> schrieb am 24.12.04
>>Well, here's an example, of the top of my head. A weakly referencing 
>>list could be useful for implementing the observer pattern in a 
>>situation where you don't want the listeners to be strongly referenced 
>>by the event source. If the event source uses such a list to hold its 
>>listeners, then as long as the listeners remain strongly referenced, 
>>they will continue to receive events from the source. There will be no 
>>need to explicitly unregister the listeners, as once they become weakly 
>>referenced, then they will effectively be removed from the event 
>>listener list automatically.
>Thank you Chris, this is exactly what I was thinking of :). With such a List, it is possible
to protect developers against creating memory leaks unintentionally, when they forget to invoke
the accordant removeListener(...) method (assuming the list is strongly referenced by a long-living
object). I think there is an Antipattern called "Lapsed Listeners Leak" that describes this
I can think of a number of uses for a weakly referencing Set, too. In 
fact, one example involves a class already in collections that could  be 
modified to use such a Set. The UniquePredicate class is a Predicate 
implementation whose evaluate() method returns true the first time it 
encounters any given input object, and returns false whenever the same 
object is encountered again. The current implementation retains each 
evaluated object in an internal Set. The evaluate method returns true if 
an input object is not in the set (the object is added to the set, in 
this case), or false if the object was already in the set.

If the internal set instance was a weakly referencing Set, then the 
UniquePredicate wouldn't prevent any of the evaluated objects from being 
GCed. Of course, this does change the semantics of the UniquePredicate 
slightly. The current implementation determines if an object is being 
reevaluated on the basis of object equality, whereas weak references are 
(by their very nature) reference based. For this reason, I'm not 
suggesting that UniquePredicate actually be changed, but this does serve 
as an example of one potential use of a weakly referencing Set.

On the other hand, regarding Stephen's original comment, I've also 
struggled to come up with any non-contrived use cases for softly 
referencing Lists and Sets. I'm sure there must be some, though...


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