| HashMap | ConcurrentHashMap |
| Not Thread safe | Thread safe |
| Performance is High | Relatively Performance is low |
| While iterating other threads not allowed to modify and will not ConcurrentModificationException | While iterating other threads are allowed to modify no exceptions thrown |
| Iterator is FailFast | Iterator is fail safe |
| Null is allowed for both keys are values | Null is not allowed in both keys and values |
| Introduced with java 1.2 verion | Introduced with Java 1.6 version |