HashMap vs ConcurrentHashMap : Java Collections Interview Question

The difference between HashMap and ConcurrentHashMap is one of the frequently asked questions in technical interviews of java. As we have already discussed another pet question of the interview is how HashMap works in java. The question HashMap vs ConcurrentHashMap is asked to check whether the candidate has an understanding of the concept of concurrency. So if you are about to give an interview, then this question should not be missed. Now let us understand the differences between the two:

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HashMap vs ConcurrentHashMap


1. Thread -Safe :

ConcurrentHashMap is thread-safe that is the code can be accessed by a single thread at a time.While HashMap is not thread-safe.

2. Synchronization Method:

HashMap can be synchronized by using synchronizedMap(HashMap) method. By using this method, we get a HashMap object which is almost equivalent to the Hashtable object. So every modification is performed on Map is locked on Map object.

import java.util.*;

public class HashMapSynchronization {
    public static void main(String[] args) {
        // create map
        Map<String,String> map = new HashMap<String,String>();
        
        // populate the map
        map.put("1","ALIVE ");
        map.put("2","IS");
        map.put("3","AWESOME");
        
        // create a synchronized map
        Map<String,String> syncMap = Collections.synchronizedMap(map);
        
        System.out.println("Synchronized map :"+syncMap);
    }
}

Output:
Synchronized map :{1=ALIVE , 2=IS, 3=AWESOME}

ConcurrentHashMap synchronizes or locks on a certain portion of the Map. To optimize the performance of ConcurrentHashMap, Map is divided into different partitions depending upon the Concurrency level. So that we do not need to synchronize the whole Map Object.


3. Null keys and Null values


ConcurrentHashMap does not allow even a single null key and a null value. So the key and the value both can not be null in ConcurrentHashMap. If you try to add a null key or a null value in the ConcurrentHashMap then it will throw NullPointerException.

While in HashMap, it allows a maximum of one null key and any number of null values.

4. Performance

In a multi-threaded environment, ConcurrentHashMap is more scalable and usually faster than synchronizedHashMap. In a single-threaded environment, HashMap performs slightly better than ConcurrentHashMap.

5. Introduction to Java

HashMap is added to the Java collections framework in Java2(JDK 1.2). On the other hand, ConcurrentHashMap is introduced in Java5(JDK 1.5).

6. Fail-fast and fail-safe

The iterators returned by ConcurrentHashMap are fail-safe in nature whereas the iterators returned by HashMap are fail-fast in nature. You can find the difference between fail-fast and fail-safe in detail here.

7. When to use ConcurrentHashMap over HashMap

ConcurrentHashMap is synchronized hence it is best suited for a concurrent multi-threaded environment. HashMap is not synchronized hence it is best suited for a single-threaded environment.

Recap : Difference between HashMap and ConcurrentHashMap

	HashMap	ConcurrentHashMap
Thread-safe	No	Yes
Synchronized	No	Yes
Null keys and Null values	One null key, any number of null values	No null keys and no null values
Performance	Perform better in single-threaded applications	Perform better in concurrent multi-threaded applications
Nature of Iterator	Fail-fast	Fail-safe
Introduction to Java	Java 2(1.2)	Java 5(1.5)

Please write in comments in case if you have any doubts.