Comparing HashMaps in Java

Question

I have two HashMaps: FOO & BAR.

HashMap FOO is a superset of HashMap BAR.

How do I find out what 'keys' are missing in HashMap BAR (i.e. exists in FOO but not BAR)?

Answer 1

Set missing = new HashSet(foo.keySet());
missing.removeAll(bar.keySet());

Answer 2

If you're using google-collections (and realistically I think it should be on the classpath of more or less every non-trivial Java project) it's just:

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