What is one way a Hashtable differs from a HashMap?

A Hashtable and a HashMap are both used to store key-value pairs, but a key difference is that Hashtable does not allow for null keys or null values. This restriction stems from the design of Hashtable, where it was built before the Java Collections Framework was established and tends to favor stricter rules regarding its content. In contrast, HashMap allows one null key and multiple null values, making it more flexible in scenarios where nulls are relevant.

The restriction on null keys in Hashtable can affect how it is used in applications where null values might need to be represented. Thus, this aspect is significant when deciding which data structure to use, especially when dealing with keys that could potentially be null. The trend in modern Java programming often favors HashMap for its performance and flexibility, particularly in environments where null might be a common state to handle.