Explain hash map internals and edge cases

You receive a knowledge check on hash maps, also called hash tables. Walk through how a standard in-memory hash map behaves and how you would think about the following edge cases:

• Approaches to resolving collisions and the trade-offs between them.
• The load factor, when resizing or rehashing happens, and the amortized cost.
• How deletions are handled, especially in open addressing.
• Worst-case behavior and ways to reduce its impact.
• The order of iteration and why it can change.
• Concurrency problems with reads and writes, plus common techniques for thread safety. Assume this is a general-purpose language runtime hash map, such as one like Java's or C++'s, rather than a distributed store. Overview: It tests your grasp of hash map internals along with related skills in data structures, algorithmic analysis, and concurrent programming under the Software Engineering Fundamentals area, covering collision resolution, load factor and resizing, deletion behavior, worst-case performance, iteration order, and thread-safety issues.