Difference Between HashMap and ConcurrentHashMap

HashMap Vs. ConcurrentHashMap: Find the Difference Between HashMap and ConcurrentHashMap

Both of these are classes in Java, but there is a fundamental difference between HashMap and ConcurrentHashMap. Here, HashMap is a type of class that comes under Traditional Collection, while ConcurrentHashMap is a type of class that comes under Concurrent Collections. There are more ways in which both of these differ, for instance:

  • HashMap isn’t thread-safe at all. Thus, it is non-synchronized in nature. The ConcurrentHashMap, on the other hand, is thread-safe.
  • Due to non-synchronization, the performance of HashMap is relatively higher, and various threads are capable of performing simultaneously. A similar case is not possible with ConcurrentHashMap. Its performance gets comparatively lower because some of the threads need to wait.
  • If the other threads try to modify or add the contents to an object while a thread iterates the HashMap object, we receive a run-time exception that says ConcurrentModificationException. On the other hand, the ConcurrentHashMap doesn’t generate any such exception when we perform any kinds of moderations during iteration.

Difference Between HashMap and ConcurrentHashMap

Here is a list of the differences between HashMap and ConcurrentHashMap.

Parameters HashMap ConcurrentHashMap
Thread Safe It is not at all thread-safe. It always remains thread-safe.
Synchronization It does not stay synchronized because it is not thread-safe in nature. It stays synchronized because it is thread-safe in nature.
Null Values It allows the keys and values to be null. It never allows a null value or key. In such a case, it will throw the NullPointerException.
Type of Iterator The iterator in HashMap is fail-fast. In case a modification happens (concurrently) during iteration, the ArrayList throws an exception named ConcurrentModificationException. It is pretty much fail-safe. Thus, a ConcurrentHashMap never throws any such exceptions during its iteration.
Performance It is comparatively faster in performance than the ConcurrentHashMap. It is comparatively much slower in performance as compared to the HashMap.
Occurrence since Java version Version 1.2 Version 1.5

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