While working on the ObjectData interface for SL Common, I wanted to have a central method to request a new resource, i.e. a new file name with a unique index - this lead to the new method OPCPackage.getUnusedPartIndex(...) As I had to work with PackagePartNames, I've realized that the current implementation carries around the errorFlag a lot instead of an appropriate control flow - so I also eliminated that. And in PackagePartCollection its better to use a sorted TreeMap.
applied via r1819708