void (String[] args) { class O {} Map<O, String> someData = new HashMap<O, String>(); Set<String> set = new HashSet<String>(); Set<O> setO = new HashSet<O>(); set.removeAll(<warning descr="'Set<String>' may not contain objects of type 'O'">someData.keySet()</warning>); setO.removeAll(someData.keySet()); }