Difference between HashMap and ConcurrentHashMap in Java? Example

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HashMap vs ConcurrentHashMap in Java
What is the difference between an HashMap and ConcurrentHashMap in Java is one of the common interview questions and knowing the answer is not just important for interviews but also for writing robust and high performance Java cocd. 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 and high performance 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 questions, mostly asked on senior level Java programmers.


Difference between HashMap and ConcurrentHashMap in Java

In 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. Thread Safety

As I said the earlier first significant difference between HashMap and ConcurrentHashMap is that later is thread-safe and can be used in a concurrent environment without external synchronization. Though it doesn't provide the same level of synchronization as achieved by using Hashtable but it's enough for the most practical purpose.

2. Synchronization

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 the whole Map.





3. Scalability

ConcurrentHashMap is more scalable and performs better than Synchronized HashMap in the multi-threaded environment while in Single threaded environment both HashMap and ConcurrentHashMap gives comparable performance, where HashMap only slightly better.


In Summary the main difference between ConcurrentHashMap and HashMap in Java Collection turns out to be thread-safety, Scalability, and Synchronization, and every Java developer should remember these different to use both of them correctly.

I think ConcurrentHashMap is a better choice than synchronized HashMap if you are using them as cache, which is the most popular use case of a Map in Java application. ConcurrentHashMap is more scalable and outperforms when a number of reader threads outnumber the number of writer threads.


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9 comments:

  1. you have not explained how it allows ? what is the internal difference. ?

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  2. you have not explained how it allows ? what is the internal difference. ?

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  3. the main difference between concurrent hash map and hashtable is , concurrent hashmap is not advicable to use when no of write operations is more then read, because it locks puts lock only in effected code not in entire block. so suppose u have 2 threada one is updating the map and another one is reading so some times u might get null even first thread has updated the map bcz of partial lock

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  4. Could you please provide some sample code for these two classes?

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  5. ConcurrentHashMap is better than HashTable because at the time of iteration it locks only particular iteration and rest iterations are free for other threads.

    Performance wise ConcurrentHashMap is better because it prevent the concurrent modification error on multithreaded Java programming.

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  6. share sample code for both with differences

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  7. final ConcurrentHashMap resultMap = new ConcurrentHashMap<>();
    and
    final HashMap resultMap = Collections.synchronizedMap(new HashMap<>());

    rest, from the programming perspective, is same as a map.
    use of Collections.synchronizedMap() can be done anywhere in your code on a map.

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  8. Also ConcurrentHashMap have method called as putIfAbsent() which is not available in HashMap

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    Replies

    1. Good point but from JDK 8 onwards putIfAbsent() and other useful methods are moved to java.util.Map interface as default methods, hence it's available to both HashMap as well as ConcurrentHashMap in Java.

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