How can I return the difference between two lists?

You can convert them to Set collections, and perform a set difference operation on them.

Like this:

Set<Date> ad = new HashSet<Date>(a);
Set<Date> bd = new HashSet<Date>(b);
ad.removeAll(bd);

If you only want find missing values in b, you can do:

List toReturn = new ArrayList(a);
toReturn.removeAll(b);

return toReturn;

If you want to find out values which are present in either list you can execute upper code twice. With changed lists.

I was looking similar but I wanted the difference in either list (uncommon elements between the 2 lists).

Let say I have:

List<String> oldKeys = Arrays.asList("key0","key1","key2","key5");
List<String> newKeys = Arrays.asList("key0","key2","key5", "key6");

And I wanted to know which key has been added and which key is removed i.e I wanted to get (key1, key6)

Using org.apache.commons.collections.CollectionUtils

List<String> list = new ArrayList<>(CollectionUtils.disjunction(newKeys, oldKeys));

Result

["key1", "key6"]