Not in the standard libraries, as far as I know. A quick and hopefully-not-too-dirty solution is:
It would probably be nicer to instead make this a decorator that could wrap any Map. A quick way to achieve that is by making a concrete implementation of
org.apache.commons.collections.map.AbstractMapDecorator, very much like the code above. Note that there are other ways to change the data in a Map besides put(), and you will need to decide whether it's necessary to guard against
all possible illicit changes to the data, or just the most common ones (via put()). Here's one loophole:
Plugging little holes like this one may take a bit more work and analysis, but it may not really be necessary for your application. Depending on how you intend to use it, and how much time you want to spend on it.