Differences between HashMap and Hashtable

There are several differences between HashMap and Hashtable in Java:

1. Hashtable is synchronized, whereasHashMapis not. This makesHashMapbetter for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.

2. Hashtabledoes not allownullkeys or values.HashMapallows onenullkey and any number ofnullvalues.

3. One of HashMap's subclasses isLinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out theHashMapfor aLinkedHashMap. This wouldn't be as easy if you were usingHashtable.

Since synchronization is not an issue for you, I'd recommendHashMap. If synchronization becomes an issue, you may also look atConcurrentHashMap.

The HashMap class is roughly equivalent to Hashtable, except that it is non synchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn't allow nulls).

1. HashMap does not guarantee that the order of the map will remain constant over time.
2. HashMap is non synchronized whereas Hashtable is synchronized.
3. Iterator in the HashMap is fail-safe while the enumerator for the Hashtable is not and throw ConcurrentModificationException if any other Thread modifies the map structurally by adding or removing any element except Iterator's own remove() method. But this is not a guaranteed behavior and will be done by JVM on best effort.

Hash table runtime complexity (insert, search and delete)