How does HashMap lookup work in Java?

Explains Java HashMap lookup through hashing, buckets, equality, collisions, resizing, and stable keys without implementation trivia.

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TL;DR

A HashMap uses a key's hash to select a bucket, then compares candidate keys in that bucket using equality. A good hash distribution keeps buckets small, so get and put are expected constant-time operations. Collisions are valid because different keys can have the same hash; equality distinguishes them. If two keys are equal, they must produce the same hash code or lookup can search the wrong bucket.

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Hash-based lookup, collision handling, equality contracts, expected complexity, and diagnosis of missing keys.

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Overview
A useful explanation connects the public key contract to the lookup process and its common failure modes.
A HashMap uses a key's hash to select a bucket, then compares candidate keys in that bucket using equality. A good hash distribution keeps buckets small, so get and put are expected constant-time operations. Collisions are valid because different keys can have the same hash; equality distinguishes them. If two keys are equal, they must produce the same hash code or lookup can search the wrong bucket.
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The map grows and redistributes entries when its capacity policy requires more buckets, so sizing can matter for a large known workload but is rarely the first concern. A mutable key is dangerous: if a field used by or changes after insertion, the entry remains in its old bucket and may no longer be found. keys and values are allowed by , but a null result from cannot distinguish a missing key from a key mapped to null; can.

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Saying collisions mean two keys overwrite each other.
Assuming hash equality implies object equality.
Changing equality-significant key fields after insertion.
Treating expected constant time as an unconditional guarantee.

How does HashMap lookup work in Java?

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