I have a cache class which contains a volatile HashMap<T> to store cache items.
I'm curious what would be the consequences of changing volatile HashMap to ConcurrentHashMap?
Would i gain performance increase? This cache is readonly cache.
What would be the best option to use? just HashMap? Cache is being populated on a interval.
First, it appears you don't understand what the volatile keyword does. It makes sure that if the reference value held by the variable declared volatile changes, other threads will see it rather than having a cached copy. It has nothing to do with thread-safety in regard to accessing the HashMap
Given that, and the fact that you say the HashMap is read-only ... you certainly don't need to use anything that provides thread-safety including a ConcurrentHashMap
Edit to add: Your last edit you now say "The cache is being populated on a interval"
That's not read-only then, is it?
If you're going to have threads reading from it while you are writing (updating the existing HashMap) then you should use a ConcurrentHashMap, yes.
If you are populating an entirely new HashMap then assigning it to the existing variable, then you use volatile