FULL PRODUCT VERSION :
java version "1.8.0_45"

ADDITIONAL OS VERSION INFORMATION :
Debian 3.16.7-ckt7-1 (2015-03-01) x86_64 GNU/Linux


A DESCRIPTION OF THE PROBLEM :
When I use the groupingBy collector with a map supplier that returns a map with some default values then it throws the following exception:

 Caused by: java.lang.ClassCastException: java.lang.Long cannot be cast to [J
at java.util.stream.Collectors$$Lambda$15/848337072.accept(Unknown Source) ~[na:na]
at java.util.stream.Collectors.lambda$groupingBy$187(Collectors.java:909) ~[na:1.8.0_45]
at java.util.stream.Collectors$$Lambda$19/1435113496.accept(Unknown Source) ~[na:na]
at java.util.stream.Collectors.lambda$groupingBy$187(Collectors.java:909) ~[na:1.8.0_45]
at java.util.stream.Collectors$$Lambda$19/1435113496.accept(Unknown Source) ~[na:na]
at java.util.stream.Collectors.lambda$groupingBy$187(Collectors.java:909) ~[na:1.8.0_45]
at java.util.stream.Collectors$$Lambda$19/1435113496.accept(Unknown Source) ~[na:na]
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) ~[na:1.8.0_45]
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[na:1.8.0_45]
at java.util.Iterator.forEachRemaining(Iterator.java:116) ~[na:1.8.0_45]
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801) ~[na:1.8.0_45]
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512) ~[na:1.8.0_45]
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502) ~[na:1.8.0_45]
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[na:1.8.0_45]
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:1.8.0_45]
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[na:1.8.0_45]



REPRODUCIBILITY :
This bug can be reproduced always.