Hash Collision in Java

Short summary

Mutable HashMap keys cause data corruption: when a key's hashCode() changes after insertion, HashMap can no longer locate it in the correct bucket because its location is calculated fresh on every lookup. Retrieval fails and returns null even though the object exists in the map, creating orphaned entries and logical duplicates if you reinsert. The fix is straightforward: always use immutable keys—String, Integer, UUID, or custom classes with final fields—to guarantee hashCode() never changes.

• •HashMap uses hashCode() to calculate bucket position; mutable keys break this contract when they change state
• •Modifying a key after insertion orphans it in the wrong bucket, making retrieval fail and returning null
• •Solution: use only immutable keys (String, Integer, UUID, final custom classes) to ensure stable hashCode()

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