Accumulate should not call getResult() more then necessary

For this rule:
rule "roomCapacityTooSmall" // TODO it takes 50-90% of the performance of drools-solver
when
$period : Period();
$room : Room($capacity : capacity);
$totalStudentSize : Number(intValue > $capacity) from accumulate(
Exam(period == $period, room == $room, $studentSize : topicStudentSize),
sum($studentSize)
);
...
end

Outside drl, I update 2 out of 700 Exam objects and call fireAllRulesAndCalculateDecisionScore.
The drl never changes an Exam. Period and Room never change.

That results into this log:
Breakpoint reached at org.drools.solver.core.evaluation.EvaluationHandler.fireAllRulesAndCalculateDecisionScore(EvaluationHandler.java:90)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.reverse(SumAccumulateFunction.java:62)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.accumulate(SumAccumulateFunction.java:53)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.reverse(SumAccumulateFunction.java:62)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.accumulate(SumAccumulateFunction.java:53)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.reverse(SumAccumulateFunction.java:62)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.accumulate(SumAccumulateFunction.java:53)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.reverse(SumAccumulateFunction.java:62)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.accumulate(SumAccumulateFunction.java:53)
Breakpoint reached at org.drools.base.accumulators.SumAccumulateFunction.getResult(SumAccumulateFunction.java:70)

There are 4 accumulate calls and 4 reverse calls. That might be accurate.
There are 8 getResult calls, that should only be 1.

Also, the getResult method creates a new Double instance each time:
return new Double( data.total );
It might be better to do
return Double.valueOf(data.total);
to allow the JVM to hot spot optimize it.