Difference between Hash‐map and HashTable

HashMap and Hashtable are both implementations of the Map interface in Java, but they have some differences, mainly in terms of synchronization, null values, and performance.

1. Synchronization:

   • Hashtable is synchronized, which means it is thread-safe. All of its methods are synchronized, making it suitable for use in a multi-threaded environment.
   • HashMap is not synchronized by default. If multiple threads access a HashMap concurrently and at least one of the threads modifies the map structurally (e.g., by adding or removing a key), it must be synchronized externally or use Collections.synchronizedMap(map) to create a synchronized version.

2. Null values:

   • Hashtable does not allow null keys or values. If you attempt to insert a null key or value, it will throw a NullPointerException.
   • HashMap allows one null key and multiple null values. It does not throw an exception if you insert a null key, but it will handle null values without any issues.

3. Performance:

   • Due to the synchronization overhead, Hashtable may be slower than HashMap in a single-threaded environment.
   • HashMap generally has better performance because it is not synchronized by default. If thread safety is not a concern, and you are in a single-threaded environment, HashMap is usually preferred.

4. Iterating over elements:

   • Both HashMap and Hashtable allow you to iterate over the elements, but the order in which the elements are returned is not guaranteed. If you need to be ordered traversal, you can use LinkedHashMap.

In summary, if you need thread safety and can't tolerate null keys or values, you might prefer Hashtable. If you are working in a single-threaded environment or can handle synchronization externally, and null values are acceptable, then HashMap is generally the more efficient choice. Keep in mind that as of my last knowledge update in January 2022, newer versions of Java may have introduced additional changes or improvements.