ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>(); map.put("key1", 1); map.put("key2", 2); int value = map.get("key1"); map.put("key2", 3); ConcurrentLinkedQueue<String> queue = new ConcurrentLinkedQueue<>(); queue.offer("element1"); queue.offer("element2"); String element = queue.poll(); CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>(); list.add("element1"); list.add("element2"); for (String element : list) { System.out.println(element); }