Explain hash map internals and edge cases

You are asked a “knowledge check” about hash maps (hash tables).

Discuss how a typical in-memory hash map works and how you would reason about these edge cases:

• Collision handling strategies and their trade-offs.
• Load factor, resizing/rehashing, and amortized complexity.
• Handling deletions (especially in open addressing).
• Worst-case behavior and how to mitigate it.
• Iteration order and how it can change.
• Concurrency concerns (reads/writes) and common approaches to thread safety.

Assume a general-purpose language runtime hash map (e.g., Java/C++-like), not a distributed store.