The java.util.ConcurrentHashMap is one of the most important classes of JDK. It was introduced in JDK 1.5 along with other concurrent collection classes like CopyOnWriteArrayList and BlockingQueue. Ever since then, it has been a workhorse in concurrent Java applications. It won't be an exaggeration If I say there is hardly any concurrent Java application that has not used ConcurrentHashMap yet. The class was another implementation of java.util.Map and a popular hash table data structure with concurrency inbuilt. This means you can also pass a ConcurrentHashMap to a method that is expecting a java.util.Map object. It was made scalable by dividing the whole map into a different segment, known as concurrency level, and then allowing threads to modify segments concurrently.
