java.lang.UnsupportedOperationException when combining two Sets

If you look at the docs of the HashMap#keySet() method, you'll get your answer(emphasis mine).

Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Therefore, you need to create a new set and add all the elements to it, instead of adding the elements to the Set returned by the keySet().

The result of keySet() does not support adding elements to it.

If you are not trying to modify hashMap1 but just want a set containing the union of the two maps' keys, try:

Set<String> mySet = new HashSet<String>();
mySet.addAll(hashMap1.keySet());
mySet.addAll(hashMap2.keySet());

Doesn't support by nature of Set which is from map.keySet(). It supports only remove, removeAll, retainAll, and clear operations.

Please read documentation