What is the difference between HashMap and HashTable?

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**HashMap** — not thread-safe, allows null. **HashTable** — thread-safe, no null. Use HashMap or ConcurrentHashMap. Analogy: **HashMap** = fast unlocked cabinet. **HashTable** = locked cabinet (safe but slow). **HashMap:** - Not thread-safe. - Allows null key/values. - Uses hashing + trees (Java 8+). - Modern (Java 1.2+). ```java HashMap map = new HashMap<>(); map.put("Alice", 90); map.put(null, 0); // OK map.put("Bob", null); // OK Integer score = map.get("Alice"); // Java 8+: map.getOrDefault("Eve", 0); map.putIfAbsent("Alice", 100); map.computeIfAbsent("Frank", k -> k.length() * 10); // Custom sizing: new HashMap<>(64, 0.5f); ``` **HashTable:** - Thread-safe (synchronized). - No null keys/values. - Legacy (Java 1.0). - Slower. ```java Hashtable table = new Hashtable<>(); table.put("Alice", 90); // table.put(null, 0); // NullPointerException! Integer score = table.get("Alice"); Enumeration keys = table.keys(); while (keys.hasMoreElements()) { String key = keys.nextElement(); System.out.println(key + " = " + table.get(key)); } ``` **Differences:** - **HashMap** — not thread-safe. **HashTable** — thread-safe. - **HashMap** — nulls OK. **HashTable** — no nulls. - **HashMap** — Iterator. **HashTable** — Enumeration. **Better alternative:** ```java ConcurrentHashMap map = new ConcurrentHashMap<>(); map.put("Alice", 90); map.compute("Alice", (k, v) -> v + 5); // Thread-safe ``` **Never use HashTable** — use HashMap or ConcurrentHashMap.

Java: What is the difference between HashMap and HashTable?