How does HashMap work internally in Java?

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A HashMap stores key-value pairs in an internal array of buckets , indexed by the key's hash. Hashing — key.hashCode() is rehashed and modulo-mapped to a bucket index. Collision handling — entries with the same bucket form a linked list ; if it grows past 8 nodes it becomes a balanced tree (Java 8+) for O(log n) lookup. Load factor & resize — default load factor 0.75 ; when exceeded the table doubles and entries are rehashed. equals + hashCode contract — equal keys must produce the same hash, otherwise lookups silently fail. Not thread-safe — concurrent writes can corrupt the structure; use ConcurrentHashMap . Know the Java 8 treeification (linked list -> tree at threshold 8). The resize mechanism (double capacity, rehash all) explains why initial capacity matters. The mutable key bug is a classic interview scenario.