When should you reach for a concurrent or copy-on-write collection instead of synchronizing a plain one?

Collections.synchronizedMap wraps every method in one lock, which serializes all access and still requires manual external synchronization for compound actions and iteration, whereas ConcurrentHashMap uses fine-grained locking and lock-free reads to scale under contention and offers atomic compute and merge operations. CopyOnWriteArrayList suits read-mostly collections like listener lists where writes are rare, since each write copies the whole array. The trade-off is that copy-on-write is terrible for write-heavy workloads, and ConcurrentHashMap's iterators are weakly consistent rather than snapshot-consistent.