What are the differences between a HashMap and a Hashtable in Java?

In Java, both HashMap and Hashtable are used to store key-value pairs, but there are several differences between them:

1. Thread Safety: Hashtable is thread-safe, meaning it is synchronized and can be safely accessed by multiple threads concurrently. On the other hand, HashMap is not thread-safe by default. If multiple threads access a HashMap concurrently, and at least one thread modifies it structurally (e.g., adding or removing entries), external synchronization is needed to ensure thread safety. However, you can use the ConcurrentHashMap class, which provides concurrent access without the need for external synchronization.
2. Null Values and Keys: HashMap allows null values and a single null key. In contrast, Hashtable does not allow null values or null keys. If you attempt to insert a null value into a Hashtable, it will throw a NullPointerException.
3. Performance: Due to the thread-safety overhead in Hashtable, HashMap generally performs better in single-threaded environments. HashMap is not synchronized by default, making it more efficient for situations where synchronization is not required.
4. Iterators: The iterators provided by HashMap are fail-fast, meaning they will throw a ConcurrentModificationException if the underlying collection is modified while iterating. In contrast, the iterators of Hashtable are not fail-fast.
5. Legacy: Hashtable is a legacy class that was part of the original Java collections framework. It is still supported for backward compatibility, but new code should generally use HashMap or other newer collections classes.

Given the above differences, HashMap is commonly used in most scenarios due to its better performance and flexibility. However, if you require thread safety or need to work with legacy code, Hashtable can be used. Alternatively, you can consider using ConcurrentHashMap if you require concurrent access and thread safety without the need for external synchronization.