Kotlinmedium

Extension functions — what are they and what is a common pitfall?

Answer

They let you add functions to a type without modifying its source (syntactic sugar). A common pitfall: extensions are resolved statically by the declared type, so they don’t behave like virtual overrides.

open class Base
class Child : Base()

fun Base.say() = "base"
fun Child.say() = "child"

val x: Base = Child()
println(x.say()) // "base" (static dispatch)

Advanced answer

Deep dive

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

Context (tags): extensions, dispatch, kotlin-basics
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

Here’s an additional example (building on the short answer):

open class Base
class Child : Base()

fun Base.say() = "base"
fun Child.say() = "child"

val x: Base = Child()
println(x.say()) // "base" (static dispatch)

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.

Interview follow-ups

When would you choose an alternative and why?