Java 8 Collectors.toMap SortedMap

I don’t think you can get much better than this:

.collect(Collectors.toMap(keyMapper, valueMapper,
                        (v1,v2) ->{ throw new RuntimeException(String.format("Duplicate key for values %s and %s", v1, v2));},
                        TreeMap::new));

where the throw lambda is the same as throwingMerger() but I can’t directly call that since it’s package private (you can of course always make your own static method for that like throwingMerger() is. )