How Does ConcurrentHashMap Ensure Thread Safety in Java?

When you’re building a Java program that uses multiple threads, things can get messy fast — especially when those threads try to read and write to the same data.

That’s where ConcurrentHashMap comes in.

It’s a special type of map designed for safe and fast access when multiple threads are working at the same time.

In this article, we’ll go over:

• What thread safety means
• Why ConcurrentHashMap is better than HashMap in multi-threaded code
• How it actually works under the hood
• A few real-world examples
• And how it helps avoid the dreaded ConcurrentModificationException

What’s the Problem With HashMap in Multithreading?

A regular HashMap is not thread-safe.

This means if two threads try to change it at the same time — say, one is adding a key and the other is removing one — your program could crash or behave unpredictably.

It could:

• Lose data
• Show the wrong values
• Even enter an infinite loop

So how do we fix that?

You could use Collections.synchronizedMap(), which adds a lock around the whole map. But then only one thread can access the map at a time—even just to read. That slows…