HashMap vs Hashtable in Java

1. Synchronization: Hashtable is synchronized, which means it is thread-safe and can be accessed by multiple threads simultaneously without any problem. In contrast, HashMap is not synchronized, and it is not thread-safe by default. However, you can make a HashMap synchronized by using the Collections.synchronizedMap() method.
2. Null values: Hashtable does not allow null keys or values, whereas HashMap allows one null key and any number of null values.
3. Performance: HashMap is generally faster than Hashtable because it is not synchronized, and therefore does not incur the overhead of synchronization.
4. Iterator: The iterator in Hashtable is a legacy Enumeration, whereas the iterator in HashMap is fail-fast.
5. Inheritance: Hashtable is a legacy class that has been in Java since the beginning, whereas HashMap was introduced in Java 1.2. Hashtable extends Dictionary, whereas HashMap extends AbstractMap.
6. Fail-safe mechanism: The fail-safe mechanism in Hashtable is achieved by locking the entire table whenever a modification operation is performed, while in HashMap, it is achieved by creating a new copy of the underlying array and modifying it.

Summary: