What is a ConcurrentHashMap and how is it different from HashMap?

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Ashutosh Kumar Verma · Answer

Java ConcurrentHashMap and HashMap
Both ConcurrentHashMap and HashMap are implementations of the Map interface in Java, but differ significantly in their concurrency support and behavior in concurrent environments:
and 
HashMap
Not concurrentand 
HashMap is not synchronized and is not thread-safe. Multiple threads can process the
HashMap at the same time, which can lead to unexpected behavior (such as
ConcurrentModificationException) if the modifications are done simultaneously without proper ordering.
and 
Performanceand 
HashMap provides good performance for single-threaded operations. Iterating through a HashMap is usually fast, and operations like
get and put are O(1) on average.
and 
Usageand 
When concurrency is not a concern (i.e., when the map will be accessed from a single thread) and you need fast operations for add, remove, and get methods, use
HashMap.
and 
ConcurrentHashMap
Concurrentand 
ConcurrentHashMap is designed to be thread-safe without the need for external synchronization. It allows simultaneous access to the map from multiple lines without data corruption or inconsistency.
and 
Internal Structureand 
ConcurrentHashMap uses different internal structure as compared to HashMap. It divides the map into blocks (buckets), each block can be locked independently. This allows multiple threads to be read and written simultaneously in blocks.
and 
Scalabilityand 
ConcurrentHashMap is designed for high concurrency scenarios where multiple threads may need access at the same time. When multiple threads frequently run and modify the map, it provides better scalability compared to
HashMap.
and 
Performance Considerationsand 
While ConcurrentHashMap ensures thread-safety, its performance features are different from HashMap. Due to the internal synchronization mechanism, individual operations (such as
get and put) typically to be slightly slower than 
HashMap. However, it is very large under concurrent access models.
and 
Usage
Use ConcurrentHashMap when you need a thread-safe map implementation that supports high concurrency. This is appropriate for situations where multiple threads must be read and written to the map simultaneously without explicit coherence.
Example-
// HashMap example (non-concurrent)
Mapandlt;String, Integer> hashMap = new HashMapandlt;>();
hashMap.put("A", 1);
hashMap.put("B", 2);
hashMap.put("C", 3);
// ConcurrentHashMap example (concurrent)
ConcurrentMapandlt;String, Integer> concurrentHashMap = new ConcurrentHashMapandlt;>();
concurrentHashMap.put("A", 1);
concurrentHashMap.put("B", 2);
concurrentHashMap.put("C", 3);
and 
Key Difference
HashMap is a non-concurrent, non-thread-safe mapping function optimized for threaded use cases.ConcurrentHashMap is a concurrent, thread-safe mapping designed for scalable situations, allowing multiple threads to access and modify the map simultaneously without external collisions
The choice of HashMap and ConcurrentHashMap depends on the concurrency requirements of your application. If your application requires thread-safe access to the map from multiple threads, ConcurrentHashMap is an appropriate choice. If concurrency is not a concern and you prioritize performance in a single threaded environment, HashMap might be more appropriate.
and 
Also, Read: 
Explain the concept of method overriding and method overloading.

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What is a ConcurrentHashMap and how is it different from HashMap?

Ashutosh Kumar Verma

1 Answers

Java ConcurrentHashMap and HashMap

HashMap

ConcurrentHashMap

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