How does HashMap work internally in Java?
Reported in Graphcore European engineering loops. Java-specific collections question on buckets, hashing, and resize behavior.
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Often asked in Graphcore loops at European offices (London, Berlin, Amsterdam, Paris, Stockholm, Dublin, and remote EU). Prepare a clear spoken answer plus key trade-offs.
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HashMap stores key-value entries in an array of buckets. Index = (n - 1) & hash(key) after spreading high bits to reduce collisions. Collisions chain in linked lists; JDK 8+ converts long chains to balanced trees (≥8 nodes) for O(log n) worst case.
Default load factor 0.75 triggers resize (double capacity) when size exceeds threshold. Resize rehashes all entries—expensive; size buckets appropriately upfront.
equals and hashCode contract: equal keys must share hash codes. Mutable keys are dangerous—changing hash breaks lookup.
Not thread-safe—use ConcurrentHashMap for concurrency (segmented/CAS locks). Contrast with HashTable (legacy synchronized) and LinkedHashMap (insertion/access order).
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