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JavaAdvanced#collections#hashmap
JavaIntermediate#collections#concurrency
HashMap vs Hashtable vs ConcurrentHashMap?
HashMap: not synchronized, allows null key/values, fast. Hashtable: synchronized (legacy), no nulls. ConcurrentHashMap: thread-safe via segmented/bucket-level locking, no null keys/values, preferred for concurrent code.
Example
Map<String,String> m1 = new HashMap<>(); m1.put(null, "ok"); Map<String,String> m2 = new ConcurrentHashMap<>(); // thread-safe, use in multi-threaded services