Explain hash collisions and Java HashMap complexity
Company: StackAdapt
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: easy
Interview Round: Technical Screen
Answer the following about hash tables (with Java’s `HashMap` as a concrete example):
1) What is a **hash collision**? Under what conditions does it happen?
2) What are the typical time complexities for `get`/`put` in a hash table (average vs worst-case), and what factors drive the worst-case?
3) In Java `HashMap`, what happens when many keys land in the same bucket? Explain the **treeification/rebalancing** behavior and the time complexity impact.
4) What are practical ways to reduce collision risk (both in code and in key design)?
Quick Answer: 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.