Synchronized HashMap vs. Concurrent HashMap in Java

Two common choices for handling concurrent operations on key-value pairs are Synchronized HashMap and Concurrent HashMap. In this blog post, we will explore these data structures in-depth, provide examples, and compare their performance and memory characteristics, helping you decide when to use each one effectively.

Synchronized HashMap

A Synchronized HashMap is a standard java.util.HashMap wrapped with synchronization to make it thread-safe. This synchronization ensures that no two threads can modify the map concurrently, preventing data corruption.


Map<KeyType, ValueType> synchronizedMap = Collections.synchronizedMap(new HashMap<>());


Map<Integer, String> synchronizedMap = Collections.synchronizedMap(new HashMap<>());
synchronizedMap.put(1, "One");
synchronizedMap.put(2, "Two");
synchronizedMap.put(3, "Three");

Concurrent HashMap

Concurrent HashMap is part of the java.util.concurrent package and is specifically designed for concurrent operations. It offers higher performance and scalability in multi-threaded environments.


ConcurrentHashMap<KeyType, ValueType> concurrentMap = new ConcurrentHashMap<>();


ConcurrentHashMap<Integer, String> concurrentMap = new ConcurrentHashMap<>();
concurrentMap.put(1, "One");
concurrentMap.put(2, "Two");
concurrentMap.put(3, "Three");

Performance Comparison

1. Synchronization:

  • Synchronized HashMap uses external synchronization to ensure thread safety, which can lead to performance bottlenecks in highly concurrent scenarios.
  • Concurrent HashMap, on the other hand, uses internal fine-grained locking mechanisms, allowing multiple threads to read and modify the map concurrently without contention.

2. Performance:

  • Synchronized HashMap can be slower in high-concurrency scenarios due to the global lock it employs to control access.
  • Concurrent HashMap is designed for concurrent access and exhibits better performance in multi-threaded applications, making it a preferred choice for such use cases.

Memory Comparison

Both Synchronized HashMap and Concurrent HashMap store key-value pairs and have similar memory footprints. The memory used depends on the number of key-value pairs stored, the capacity of the underlying arrays, and the load factor.

When to Use Synchronized HashMap

  1. In legacy code: If you are working with older Java applications or libraries that use Synchronized HashMap, you might have to maintain compatibility.
  2. When only a few threads access the data: In scenarios with low thread contention, using Synchronized HashMap might be sufficient, and it simplifies code.
  3. When you need to convert a non-thread-safe HashMap into a thread-safe one without changing the existing code.

When to Use Concurrent HashMap

  1. In multi-threaded applications: When dealing with high-concurrency scenarios where multiple threads need to read and write data concurrently, Concurrent HashMap is the better choice.
  2. When performance and scalability are critical: In applications with a heavy load and high-performance requirements, Concurrent HashMap’s fine-grained locking provides better performance and scalability.
  3. In modern Java development: Concurrent HashMap is the recommended choice for new Java applications, as it is specifically designed for multi-threaded environments.

Synchronized HashMap and Concurrent HashMap serve the purpose of handling concurrent access to key-value pairs, but they differ significantly in terms of performance and scalability. Consider your application’s concurrency requirements and performance demands to choose between these two data structures effectively. In most modern Java applications, Concurrent HashMap is the preferred choice, offering superior performance and scalability in multi-threaded environments.

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