Currently XSSFRow implements the Comparable<XSSFRow> interface, HSSFRow doesn't declare that it implements a Comparable interface but has a compareTo(Object) method, and SXSSFRow does neither. For consistency, all three should implement a Comparable interface. To avoid Java generics craziness, the Row interface will NOT be required to extend Comparable<Row>.
Added in r1717054, updated docs in r1717055. Also changed HSSFRow.hashCode implementation to return a value, which is needed if HSSFRow.toString() is not overridden.