Concurrency level of ConcurrentHashMap
The concurrency level of ConcurrentHashMap in Java refers to the number of segments into which the hash table is divided. Each segment of the hash table acts as an independent hash table, allowing multiple threads to operate on different segments concurrently without blocking each other. The default concurrency level of Concurrent HashMap is 16, meaning it has 16 segments by default.

Table of Contents
Explanation
1. Segmentation
Concurrent HashMap uses a technique called “segmentation” to achieve concurrency. Instead of locking the entire map, it divides the map into multiple segments, and each segment is independently locked using a separate lock.
2. Reduced Contention
By dividing the map into segments, Concurrent HashMap reduces contention among threads accessing different parts of the map. This allows multiple threads to read and write to different segments concurrently, improving performance in multi-threaded scenarios.
3. Dynamic Sizing
The number of segments in a HashMap can be adjusted dynamically based on the concurrency level specified during construction. A higher concurrency level increases the number of segments, reducing contention further but potentially increasing memory overhead.
java
import java.util.concurrent.ConcurrentHashMap;
public class Concurrent HashMapExample {
public static void main(String[] args) {
// Creating a Concurrent HashMap with default concurrency level (16)
Concurrent HashMap<String, Integer> concurrent HashMap = new ConcurrentHashMap<>();
// Inserting elements into the Concurrent HashMap
concurrent HashMap.put("A", 1);
concurrent HashMap.put("B", 2);
concurrent HashMap.put("C", 3);
// Retrieving elements from the Concurrent HashMap
System.out.println("Value associated with key 'A': " + concurrentHashMap.get("A"));
System.out.println("Value associated with key 'B': " + concurrentHashMap.get("B"));
System.out.println("Value associated with key 'C': " + concurrentHashMap.get("C"));
}
}
Example
Concurrent HashMap with the default concurrency level. We insert key-value pairs into the map and then retrieve values associated with keys. Under the hood, Concurrent HashMap uses segmentation to achieve thread safety and efficient concurrency. The default concurrency level ensures a balance between concurrency and memory overhead, but it can be adjusted based on specific application requirements.