A ConcurrentNavigableMap has all the methods of a NavigableMap,
plus all the methods of a ConcurrentMap. Other NavigableMaps do not have ConcurrentMap methods.
Is it ConcurrentNavigableMap can be modified by different threads only?
No - if for some reason you want to modify a ConcurrentNavigableMap using only one
thread, that's fine, it will work normally. The point is that if you
do have multiple threads accessing a Map, you need a ConcurrentMap.