[elastic java transport client] Why Terms.Bucket is class,not interface

Hi,
I'm looking into elastic java transport
client, org.elasticsearch.search.aggregations.bucket.terms.Terms interface.
Unlike all other aggregations Terms.Bucket is class,not interface.
It has one package private method , compareTerm -- because of this method
it's impossible to implement Terms inteface.
So my question is -- was it done intentionally or it's just error and I can
submit issue?
Sorry if I post to wrong mailing list -- not sure to which exactly mailing
list should I post it, but seems not to be related to general Elastic usage.

Best Regards,
Iana