Kotlineasy

What does `by lazy` do and when is it useful?

Answer

`by lazy` computes a value on first access and then caches it. It’s useful for expensive initialization you may not need, and it can be thread-safe depending on the chosen mode.

val config by lazy { loadConfig() }

fun loadConfig(): String = "ok"

Advanced answer

Deep dive

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

Context (tags): lazy, delegates, initialization
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):

val config by lazy { loadConfig() }

fun loadConfig(): String = "ok"

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?
What production issues show up and how do you diagnose them?