HashMap vs Hashtable: Key Differences

HashMap and Hashtable are both collection classes used to store key-value pairs, but they have a few differences between them:

1. Thread safety: Hashtable is thread-safe, meaning it is safe to operate on Hashtable in a multi-threaded environment. In contrast, HashMap is not thread-safe, so additional synchronization measures are required to ensure thread safety when operating on HashMap in a multi-threaded environment.
2. The value of null: Hashtable does not allow storing null keys or values. If an attempt is made to insert a null key or value into Hashtable, a NullPointerException will be thrown. On the other hand, HashMap allows storing one null key and multiple null values.
3. Inheritance relationships: Hashtable is a thread-safe class, while HashMap is not. Hashtable is a subclass of Dictionary, while HashMap is a subclass of AbstractMap.
4. Performance: HashMap is generally faster than Hashtable because HashMap is not thread-safe and does not incur the overhead of thread synchronization. In a single-threaded environment, HashMap is usually faster than Hashtable.

In conclusion, generally speaking, if thread safety is not needed and the existence of null values is acceptable, it is recommended to use HashMap; if thread safety is required, Hashtable can be considered.