Calling PropertyClass#setNumber might have unattended side effects

It might be tempting to call com.xpn.xwiki.objects.classes.PropertyClass#setNumber to reorder properties in a class, but this is likely to create inconsistencies as the fields are stored in a LinkedHashMap which is not reordered when calling this method.
What's more, nothing prevents someone from calling setNumber with the same number for several properties in a class.

This method should probably be updated to allow this usage, or a new api to reorder fields should probably be introduced.