Kotlinmedium

Delegated properties (`by`): what problem do they solve?

Answer

Delegated properties let you reuse getter/setter logic (lazy initialization, observable changes, validation) without boilerplate. You delegate the property to another object that provides `getValue`/`setValue`.

var name: String by Delegates.observable("<init>") { _, old, new ->
  println("$old -> $new")
}

name = "Ala"

Advanced answer

Deep dive

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

Context (tags): kotlin, delegation, properties, observable
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):

var name: String by Delegates.observable("<init>") { _, old, new ->
  println("$old -> $new")
}

name = "Ala"

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?