Kotlinmedium

`lateinit` vs nullable property: when would you choose `lateinit`?

Answer

Use `lateinit var` for a non‑nullable property that will be initialized later (e.g., in DI or lifecycle), when `null` is not a valid state. It works only for mutable, non‑primitive types. You can check initialization with `::prop.isInitialized`.

Advanced answer

Deep dive

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

Context (tags): kotlin, lateinit, null-safety, properties
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 "`lateinit`-vs-nullable-property:-when-would-you-"
function explain() {
  // Start from the core idea:
  // Use `lateinit var` for a non‑nullable property that will be initialized later (e.g., in DI
}

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.