HashMap is not thread‑safe and is best for single‑threaded or externally synchronized use. ConcurrentHashMap supports safe concurrent reads/writes with better scalability; it disallows null keys/values. Use it when multiple threads access the map without external locking.
Advanced answer
Deep dive
Expanding on the short answer — what usually matters in practice:
Explain the "why", not just the "what" (intuition + consequences).
Trade-offs: what you gain/lose (time, memory, complexity, risk).
Edge cases: empty inputs, large inputs, invalid inputs, concurrency.
Examples
A tiny example (an explanation template):
// Example: discuss trade-offs for "hashmap-vs-concurrenthashmap:-when-should-you-us"
function explain() {
// Start from the core idea:
// HashMap is not thread‑safe and is best for single‑threaded or externally synchronized use.
}
Common pitfalls
Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).