java - do not understand Class<? extends VersionedProtocol> protocol; -

when reading java program, meet following piece line of code

class<? extends versionedprotocol> protocol; 

what <? extends versionedprotocol> stand for? mechanism define class?

the class class has generic type parameter represents itself. here class<? extends versionedprotocol> means class object representing versionedprotocol class or subclass of versionedprotocol.

this known upper-bounded wildcard in java generics.