What is the difference between HashMap and Hashtable?

HashMap and Hashtable are both data structures used for storing key-value pairs, but they have the following differences:

1. Thread safety: Hashtable is thread-safe, while HashMap is not. In a multi-threaded environment, when multiple threads access Hashtable simultaneously, it will automatically synchronize to ensure thread safety. On the other hand, in a multi-threaded environment, HashMap may encounter concurrent modification exceptions if additional synchronization is not implemented.
2. Empty key and value: Hashtable does not allow storing null keys or null values, trying to do so will result in a NullPointerException. In contrast, HashMap allows for storing one empty key and multiple empty values.
3. Iterators: The iterator of Hashtable is fail-fast, meaning that if another thread makes structural modifications (such as adding or deleting elements) to the Hashtable during iteration, it will throw a ConcurrentModificationException. On the other hand, the iterator of HashMap is not fail-fast and allows elements to be added or removed during iteration.
4. Inheritance: Hashtable is implemented based on the Dictionary class, while HashMap is implemented based on the AbstractMap class. Due to Dictionary being an older class, it is recommended to use HashMap starting from Java 2.

Overall, HashMap performs better than Hashtable in terms of efficiency, but in a multithreaded environment, Hashtable is recommended for thread-safe operations. In a single-threaded environment, HashMap is more commonly used.