HDFS-2492 has identified a need for DNSToSwitchMapping implementations to provide a bit more topology information (e.g. whether or not there are multiple switches). This could be done by writing an extended interface, querying its methods if present and coming up with a default action if there is no extended interface.
Alternatively, we have a base class that all the standard mappings implement, with a boolean isMultiRack() method; all the standard subclasses would extend this, as could any third party topology provider. The advantage of this approach is that it is easier to add new operations without going into a multi-interface mess.