What is the main difference between HashMap and Hashtable?

The main distinction between HashMap and Hashtable is that HashMap allows null values (both keys and values), whereas Hashtable does not permit null keys or values at all. This flexibility in HashMap can be particularly useful when you need to represent a mapping that can include unspecified or absent values, as null can be used as a key or value to indicate such cases.

Hashtable, having been a part of Java from earlier versions, was designed with a more rigid structure. It was synchronized from the beginning to ensure thread safety, but this comes at the cost of performance, especially in a single-threaded context.

While there are other differences between the two, such as synchronization and performance characteristics, the significant and immediate contrast lies in their handling of null, which can greatly affect how they are used in practical programming scenarios.