ConcurrentHashMap in Java is introduced as an alternative of Hashtable in Java, which is a synchronized collection class, that makes the main difference between HashMap and ConcurrentHashMap which is one is non synchronized , non thread safe and not for use in Concurrent multi-threaded environment while ConcurrentHashMap is a thread-safe collection and intended to be used as primary Map implementation especially for multi-threaded and Concurrent environment. Apart from thread-safety there are some subtle differences between HashMap and ConcurrentHashMap which we will see in this article. By the way Difference between HashMap and ConcurrentHashMap as well as ConcurrentHashMap vs Hashtable are two popular core Java interview question, mostly asked on senior level Java programmers.
Difference between HashMap and ConcurrentHashMap in JavaIn this section we will see some more details about HashMap and ConcurrentHashMap and compare them on various parameters like thread-safety, synchronization, performance, ease of use etc.
1) As I said earlier first significant difference between HashMap and ConcurrentHashMap is that later is thread-safe and can be used in concurrent environment without external synchronization. Though it doesn't provide same level of synchronization as achieved by using Hashtable but its enough for most practical purpose.
2)You can make HashMap synchronized by wrapping it on Collections.synchornizedMap(HashMap) which will return a collection which is almost equivalent to Hashtable, where every modification operation on Map is locked on Map object while in case of ConcurrentHashMap, thread-safety is achieved by dividing whole Map into different partition based upon Concurrency level and only locking particular portion instead of locking whole Map.
3) ConcurrentHashMap is more scalable and performs better than Synchronized HashMap in multi-threaded environment while in Single threaded environment both HashMap and ConcurrentHashMap gives comparable performance, where HashMap only slightly better.
In Summary Main difference between ConcurrentHashMap and HashMap in Java Collection turns out to be thread-safety, Scalability and Synchronization. ConcurrentHashMap is better choice than synchronized HashMap if you are using them as cache, which is most popular use case of a Map in Java application. ConcurrentHashMap is more scalable and outperform when number of reader threads outnumber number of writer threads.