HashMap — unsynchronized map allowing null keys/values. HashTable — synchronized legacy map, no nulls.Vivid analogy: HashMap is a personal whiteboard — quick, but concurrent writes could cause confusion. HashTable is a locked ledger — safe but slower.Why it matters: HashMap is the workhorse of Java collections. HashTable is legacy from Java 1.0 — use ConcurrentHashMap for thread-safe scenarios.javaHashMap<String, Integer> hm = new HashMap<>();hm.put("apple", 3);hm.put(null, 0); // null key OKhm.put("banana", null); // null value OKHashtable<String, Integer> ht = new Hashtable<>();ht.put("apple", 3);// ht.put(null, 0); // NullPointerException!ConcurrentHashMap<String, Integer> cm = new ConcurrentHashMap<>();cm.put("apple", 3);cm.merge("apple", 1, Integer::sum); // atomic: 4- HashMap — single-threaded (default).- HashTable — legacy, avoid.- ConcurrentHashMap — thread-safe modern choice.