In Java, both HashMap and Hashtable are used to store key-value pairs, but there are some key differences between them:
Thread-safety: Hashtable is thread-safe, meaning that it can be safely accessed and modified by multiple threads simultaneously. On the other hand, HashMap is not thread-safe and should be used in a single-threaded environment or when thread-safety is not required. To make HashMap thread-safe, it can be wrapped using the Collections.synchronizedMap() method.
Null values: Hashtable does not allow null keys or values, whereas HashMap allows one null key and any number of null values.
Performance: HashMap is generally faster and more efficient than Hashtable due to its lack of synchronization. However, if thread safety is required, Hashtable may be a better choice.
Iteration: The iterator of Hashtable is a fail-safe iterator, which means that it will not throw a ConcurrentModificationException if the table is modified during iteration. The iterator of HashMap is a fail-fast iterator, which will throw a ConcurrentModificationException if the table is modified during iteration.
Here is an example to demonstrate the usage of both classes:
Hashtable<String, Integer> table = new Hashtable<>();
HashMap<String, Integer> map = new HashMap<>();
In summary, HashMap and Hashtable are similar in functionality, but Hashtable is thread-safe and does not allow null keys or values, while HashMap is not thread-safe and allows null keys and values. The choice of which one to use depends on the requirements of the application, such as whether thread safety is required or null values are allowed.