public static void main(String[] args) {
            ConcurrentHashMap<String, String> c = new ConcurrentHashMap<String, String>();
            c.put("1", "a");
            c.put("2", "b");
            c.put("3", "c");
            c.put("4", "d");

            // The following code won't fail
            for (Map.Entry<String, String> entry : c.entrySet()) {
                c.put("5", "e");
                System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
                c.put("6", "f");
            }

            // another flavour of the above code which would fail neither

            for(String str:c.keySet()) {
                c.put("5", "e");
                System.out.println(c.get(str));
                c.put("6", "f");
            }
        }