Kotlinmedium

Generics variance: what do `out` and `in` mean in Kotlin?

Answer

`out T` means the type only produces T (you can read T), so it’s covariant (e.g., `List<out Animal>` can hold `List<Dog>`). `in T` means it only consumes T (you can pass T in), so it’s contravariant (e.g., `Comparator<in Dog>`). It prevents unsafe reads/writes.

Advanced answer

Deep dive

Expanding on the short answer — what usually matters in practice:

Context (tags): kotlin, generics, variance, type-safety
JVM: memory (heap/stack), GC, and what drives latency.
Contracts: equals/hashCode/toString, mutability and consequences.
Performance: boxing, allocations, collections, inlining.
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 "generics-variance:-what-do-`out`-and-`in`-mean-i"
function explain() {
  // Start from the core idea:
  // `out T` means the type only produces T (you can read T), so it’s covariant (e.g., `List<ou
}

Common pitfalls

Too generic: no concrete trade-offs or examples.
Mixing average-case and worst-case (e.g., complexity).
Ignoring constraints: memory, concurrency, network/disk costs.