Compare hashmaps in Java

I have two hashmaps: foo & bar

HashMap foo is a superset of HashMap bar

How to find out the missing "key" in HashMap bar (i.e. it exists in foo but not in bar)?

Solution

If you are using Google collections (and actually I think it should be on the classpath of more or less every non trivial java project), it just:

Set<X> missing = Sets.difference(foo.keySet(),bar.keySet();