Explain hash collisions and Java HashMap complexity

Q: Explain hash collisions and Java HashMap complexity

This question evaluates a candidate's understanding of hash tables, hashing and collision phenomena, algorithmic time complexity for get/put operations, Java HashMap internals including bucket behavior and treeification/rebalancing, and competency in designing keys to reduce collision risk.

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Question

Answer the following about hash tables (with Java’s HashMap as a concrete example):

What is a hash collision ? Under what conditions does it happen?
What are the typical time complexities for get / put in a hash table (average vs worst-case), and what factors drive the worst-case?
In Java HashMap , what happens when many keys land in the same bucket? Explain the treeification/rebalancing behavior and the time complexity impact.
What are practical ways to reduce collision risk (both in code and in key design)?

Explain hash collisions and Java HashMap complexity

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