I was using ArrayList before and I don't think this is the real problem. collect() will return object compatible with List interface so it is valid to use size() and get() methods on $removeUs.
I have helper method myhelper.log() with accepts any object and myhelper.log($removeUs.get(i)) works for me, unfortunately retract($removeUs.get(i)) doesn't work with error:

Error: unable to resolve method using strict-mode: org.drools.spi.KnowledgeHelper.drools()] [Near : {... drools.retract($removeUs ....}]

It looks like it is not even compiled.

2012/11/30 Wolfgang Laun <wolfgang.laun@gmail.com>
On 30/11/2012, Martin Minka <martin.minka@gmail.com> wrote:
> Thank you for the tip. What solution would you suggest to solve my problem
> ?

Look into the javadoc of java.util to find any class implementing List :-)
-W

>
> Martin
>
>
> 2012/11/30 Wolfgang Laun <wolfgang.laun@gmail.com>
>
>> You cannot use an interfact (List) to instantiate an object, which
>> is happening due to the collect.
>> --W
>>
>> On 30/11/2012, Martin Minka <martin.minka@gmail.com> wrote:
>> > I want to keep only 1 fact with id=="aaaa". But this is not working:
>> >
>> > rule "leave only one"
>> > when
>> >     $removeUs : java.util.List(size>1)
>> >         from collect(Fact(id=="aaaa")
>> > then
>> >     size = $removeUs.size();
>> >     for (int i=1; i < size; i++) {
>> >         retract($removeUs.get(i));
>> >     }
>> > end
>> >
>> _______________________________________________
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>> rules-users@lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-users
>>
>
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