Concurrent Collections and Maps 2 – Concurrency: Part II
Some collections allow the null value, and others do not. All concurrent collections, blocking queues, and concurrent maps do not allow the null value as elements, whereas the copy-on-write collections do. All collections that embody the concept of a set do not allow duplicates—for example, a CopyOnWriteArraySet. All maps (e.g., a ConcurrentHashMap) do not allow