HashMap is the Class which is under Traditional Collection and ConcurrentHashMap is a Class which is under Concurrent Collections, apart from this there are various differences between them which are: HashMap is non-Synchronized in nature i.e. HashMap is not Thread-safe whereas ConcurrentHashMap is Thread-safe in nature. HashMap performance is relatively high because it is non-synchronized in nature and any number of threads can perform simultaneously. But ConcurrentHashMap performance is low sometimes because sometimes Threads are required to wait on ConcurrentHashMap. While one thread is Iterating the HashMap object, if other thread try to add/modify the contents of Object then we will get Run-time exception saying ConcurrentModificationException.Whereas In ConcurrentHashMap we wont get any exception while performing any modification at the time of Iteration. Using HashMap. In HashMap, null values are allowed for key and values, whereas in ConcurrentHashMap null value is not allowed for key and value, otherwise we will get Run-time exception saying NullPointerException. HashMap is introduced in JDK 1.2 whereas ConcurrentHashMap is introduced by SUN Microsystem in JDK 1.5.