Why Am I Getting a java.util.ConcurrentModificationException in My HashMap Processing Code?

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Understanding ConcurrentModificationException

A ConcurrentModificationException is thrown when a thread detects that a collection has been modified concurrently during an iteration process. Essentially, it’s Java’s way of telling you that the collection you’re working with has been structurally altered in a way that could lead to unpredictable behavior.

Why Does It Happen With HashMap?

HashMap is not synchronized, meaning it isn’t thread-safe by default. When multiple threads modify the map concurrently, it can lead to a situation where one thread is iterating over the map while another modifies it. This can corrupt the state of the map, making its behavior unpredictable. Java detects this situation and throws a ConcurrentModificationException to prevent potential issues such as infinite loops or incorrect data.

Common Scenarios Leading to ConcurrentModificationException

Here are a couple of scenarios where you might encounter this exception:

Modifying during iteration:

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Using multiple threads:

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How to Avoid ConcurrentModificationException

Use Concurrent Collections

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Synchronized Block

You can manually synchronize on the map to ensure that only one thread modifies it at a time.

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Copy-on-Write Approach

For cases where iteration is more frequent than modification, you can consider creating a snapshot of the collection.

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Conclusion

Ready to write safer Java code? Start by examining where you're using HashMap and consider whether you need to adjust your approach to prevent concurrent modification issues.