java - Weird syntax of an EnumMap -

an enummap defined follow:

class enummap<k extends enum<k>,v> 

what "k extends enum<k>" mean?

i can understand example:

k extends number 

in case k type of object extend number. example integer.

but k extends enum<k> mean?

k extends enum<k> introduces type parameter k can bind sub-types of enum first-type parameter itself.