What is difference between HashMap and Hashtable in Java?

In Java, both HashMap and Hashtable are used to store and retrieve key-value pairs. However, there are some key differences between the two:

Synchronization: Hashtable is synchronized, which means it is thread-safe and multiple threads can access it concurrently without causing data inconsistencies. On the other hand, HashMap is not synchronized by default, and if you need synchronization, you can use Collections.synchronizedMap() to create a synchronized version of HashMap.

Null values: Hashtable does not allow null values for both keys and values. If you try to insert a null key or value, it will throw a NullPointerException. In contrast, HashMap allows null values and a single null key.

Performance: Since Hashtable is synchronized, it incurs the overhead of acquiring and releasing locks, making it slightly slower than HashMap. If you don't need thread-safety, using HashMap can result in better performance.

Iterator fail-fast behavior: Both HashMap and Hashtable provide fail-fast iterators, meaning if the underlying collection is modified structurally while iterating, an exception (ConcurrentModificationException) is thrown. However, the way they achieve this behavior is different. Hashtable uses a single lock for the whole table, while HashMap uses a fail-fast iterator on top of its internal data structure (bucket-array and linked list).

Inheritance: Hashtable is a subclass of Dictionary, whereas HashMap is a subclass of AbstractMap. The Dictionary class is obsolete, and it is recommended to use Map interfaces and their implementations (such as HashMap) instead.

In general, if you need thread-safety or you're working with legacy code that requires Dictionary or synchronized behavior, you can use Hashtable. If you don't need thread-safety and performance is a concern, HashMap is the preferred choice.