ConcurrentHashMap is introduced as an alternative of Hashtable in Java 1.5 version, ConcurrentHashMap is a synchronized collection class, And the main difference between HashMap and ConcurrentHashMap is HashMap is not synchronized as well as not thread safe because of that not use in Concurrent multi-threaded environment. But ConcurrentHashMap is a thread-safe collection class and we can use them in Concurrent multi-threaded environment.
Difference between HashMap and ConcurrentHashMap classes in Java are following:-
| Synchronized | HashMap is not synchronized but HashMap can be synchronized using the Collections.synchronizedMap() method | ConcurrentHashMap is synchronized. |
| Thread Safety | HashMap is not thread safe. | ConcurrentHashMap is thread safe and fit for use in a multi-threaded environment |
| Iterator type | HashMap iterator is fail-fast as it throws a ConcurrentModificationException if concurrent modification happens during iteration. | ConcurrentHashMap is fail-safe and it will never throw ConcurrentModificationException during iteration. |
| Null values | HashMap allows one key and value to be null. | ConcurrentHashMap does not allow null key/value. It will throw NullPointerException. |
| Performance | HashMap is faster because there is no synchronization | ConcurrentHashMap is slower than HashMap. |
| Introduce | HashMap is a part of Java’s collection since Java 1.2 | ConcurrentHashMap is a part of Java collection since Java 1.5 |