HashMap — unsynchronized, allows null keys/values. HashTable — synchronized, allows no nulls. Legacy class.Vivid analogy: HashMap is like a personal notebook — fast, but concurrent writes could cause issues. HashTable is like a bank ledger with a lock — safe but slower.Why it matters: HashMap is one of the most-used data structures in Java. Understanding its internals is essential for writing efficient code.javaHashMap<String, Integer> hm = new HashMap<>();hm.put("apple", 3);hm.put(null, 0); // OK — null key allowedhm.put("banana", null); // OK — null value allowedhm.get("apple"); // 3Hashtable<String, Integer> ht = new Hashtable<>();ht.put("apple", 3);// ht.put(null, 0); // NullPointerException!// ht.put("k", null); // NullPointerException!ConcurrentHashMap<String, Integer> cm = new ConcurrentHashMap<>();cm.put("apple", 3);cm.merge("apple", 1, Integer::sum); // atomic: 4- HashMap — default for single-threaded use.- HashTable — legacy, avoid in new code.- ConcurrentHashMap — modern thread-safe choice.