What a good transactional API is like is an interesting question.

   public void modify(K key, Function<Optional<V>,Optional<V>> modifier)

or something along those lines covers the race of a single value being changed out from under you, but if you want to update one or more keys you might want

   public void modifyMany(Set<K> key, Consumer<Map<K,V>> modifier)

where the modifier gets passed a modifiable view that has just the keys in the set.

There is also the Clojure-style immutable API for maps though I can say I never really enjoyed working with that the way I enjoyed immutable lists (which are Lispier than Lisp: I went through all the stages of grief reading On Lisp and couldn't help think "If he was using Clojure he wouldn't be struggling with nconc)