How does Java HashMap work internally?

Direct answer

What a strong answer covers

How it works

HashMap stores entries in an array of buckets indexed by a spread of the key’s hashCode. Keys with the same bucket form a linked list, which since Java 8 converts to a red-black tree once it holds more than eight entries, keeping worst-case lookups logarithmic instead of linear. When the number of entries exceeds capacity times the load factor of 0.75, the table doubles and every entry is rehashed. Average get and put are constant time only if hashCode distributes keys well and equals is consistent with it.

Example

Using a mutable object as a key and changing a field after insertion moves its hash but not its bucket, so map.get returns null for a key that is still inside the map. Pre-sizing a map with new HashMap<>(expected * 4 / 3 + 1) before loading 100,000 entries avoids several rehash passes during the bulk insert.

Common mistakes

• Overriding equals without hashCode, so equal keys land in different buckets.
• Expecting iteration order to be stable or insertion-ordered, which LinkedHashMap provides and HashMap does not.
• Sharing a HashMap between threads without synchronisation and corrupting it during a resize.

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