`when` can return a value (it’s an expression). “Exhaustive” means all possible cases are covered, so you don’t need an `else`. For `enum` and `sealed class`, the compiler can check exhaustiveness and will force you to handle new cases when the type changes.
Advanced answer
Deep dive
Expanding on the short answer — what usually matters in practice:
Context (tags): kotlin, when, sealed, exhaustive
JVM: memory (heap/stack), GC, and what drives latency.
Contracts: equals/hashCode/toString, mutability and consequences.
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 "`when`-as-an-expression:-what-does-it-mean-that-"
function explain() {
// Start from the core idea:
// `when` can return a value (it’s an expression). “Exhaustive” means all possible cases are
}
Common pitfalls
Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).