I read that bad implementations of equals and hashCode can result in memory issues. What causes memory leaks in Java applications when using custom objects as keys inside a HashSet or HashMap? The objects seem to multiply and never get cleaned up by the GC mechanism.
3 answers
When you insert an object into a HashMap or HashSet, its position is determined by its hashCode value. If your custom class modifies its internal state after insertion, or if its hashCode method isn't consistent, its computed hash changes. When you later try to remove or find that object, the collection looks in the wrong bucket and fails to locate it. The map keeps holding a strong reference to the original object inside the old bucket, making it impossible for the Garbage Collector to clean it up.
This makes total sense for mutable keys, but would making our custom key objects strictly immutable prevent this specific bug from happening entirely?
Using mutable objects as keys in collections is a classic anti-pattern that leads to incredibly frustrating debugging sessions.
Spot on, Diana. It is a silent killer because the application won't throw any errors; it just quietly hoards memory until deployment.
Absolutely, Scott. Making your map keys immutable using final fields and generating hashCode once ensures that the object's bucket location never shifts. Using Java Records for keys is an excellent modern approach to achieve this safety natively.