NOTE
HashMap in JDK 7
Historical JDK 7 HashMap bucket-chain implementation and resize behavior, clearly separated from modern JDK internals.
This is a historical learning note and may contain outdated or incomplete understanding.
This note describes JDK 7-era HashMap internals: an array of buckets with linked collision chains and a resize/rehash implementation specific to that generation.
HashMap is not safe for unsynchronized concurrent structural mutation. Historical JDK 7 resize races could corrupt bucket chains; this is one reason old discussions warn about loops during concurrent resize.
The durable lesson is not the exact transfer algorithm: use HashMap only with appropriate confinement/synchronization, and use ConcurrentHashMap for shared concurrent map access. Do not apply JDK 7 node/resize details to current JDKs.