Java, drilldown an ArrayList of HashMaps

Question

I have an ArrayList<HashMap<String, String>>. I'd like to quickly extract from this a new ArrayList<String> comprising all the keys.

How do I do this?

Answer 1

I suggest you do

List<String> allKeys = new ArrayList<String>();

for (Map<String, String> map : yourListOfMaps)
    allKeys.addAll(map.keySet());

If you're not interested in duplicate keys (i.e., if you don't want two identical entries in allKeys just because it exists as key in two maps) I would suggest you let allKeys be of type HashSet<String> instead.

After traversal you could do allKeysList = new ArrayList<String>(allKeys); if order is important to you in the end.