Where will you use Hashtable and where will you use HashMap

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  • of possible future changes.

In Java, both Hashtable and HashMap are implementations of the Map interface, and they are used to store key-value pairs. However, there are some differences between the two:

  1. Thread Safety:
    • Hashtable is synchronized, which means it is thread-safe. Multiple threads can safely access a Hashtable concurrently.
    • HashMap is not synchronized by default. If you need thread safety, you can use Collections.synchronizedMap() to create a synchronized version of a HashMap.
  2. Null Values:
    • Hashtable does not allow null keys or values. If you try to insert a null key or value, it will throw a NullPointerException.
    • HashMap allows one null key and any number of null values.
  3. Performance:
    • HashMap generally performs better than Hashtable because it’s not synchronized by default. If you don’t need thread safety, HashMap is often a better choice.
  4. Iterator:
    • Hashtable enumerator is not fail-fast. If the Hashtable is structurally modified at any time after the enumerator is created, it will throw a ConcurrentModificationException.
    • HashMap iterator is fail-fast. If the HashMap is structurally modified at any time after the iterator is created, it will throw a ConcurrentModificationException.

In summary:

  • Use Hashtable when you need a synchronized (thread-safe) map and you don’t want to deal with external synchronization.
  • Use HashMap when you don’t need thread safety or when you are able to manage synchronization explicitly (e.g., using Collections.synchronizedMap()).

In modern Java development, HashMap is more commonly used unless there’s a specific requirement for thread safety. If thread safety is required, alternatives like ConcurrentHashMap are often preferred over Hashtable due to better performance.