Explain the differences between HashMap and Hashtable classes and when to prefer each.

HashMap and Hashtable are implementations of the Map interface in Java, but there are key differences:

• Synchronization: Hashtable is synchronized (thread-safe), which makes it slower in single-threaded applications. HashMap is not synchronized, so it is faster but not safe for concurrent access without external synchronization.

• Support for null: HashMap allows one null key and multiple null values. Hashtable does not permit null keys or null values.

• Inheritance: Hashtable is a legacy class, inherited from Dictionary, while HashMap inherits from AbstractMap.

When to use:

• If thread safety without additional synchronization is needed, you can use Hashtable, but modern alternatives like ConcurrentHashMap are preferred.
• For most cases, especially in single-threaded applications, HashMap is preferable due to better performance and flexibility.

Example of using HashMap:

Map<String, Integer> map = new HashMap<>();
map.put(null, 1); // allowed
map.put("key", 2);