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Kotlin null-safety — what do `?`, `?.`, `?:`, and `!!` mean?

Tags

#null-safety#elvis#nullable

Back to category Practice quiz

Answer

`T?` is a nullable type, `?.` is a safe call, `?:` (Elvis) provides a default, and `!!` asserts non-null (throws if it’s null).

val name: String? = null
val len = name?.length ?: 0
// val crash = name!!.length // throws if name is null

Advanced answer

Deep dive

Kotlin encodes nullability in the type system.

`T?`: value may be null.
`?.`: safe call (returns null instead of throwing).
`?:`: Elvis (fallback when the left side is null).
`!!`: not-null assertion (throws NPE if null) — treat as a last resort.

Practical patterns

Prefer safe calls + Elvis:

val len = name?.length ?: 0

Use `let` to work only when non-null:

name?.let { println(it.length) }

Use `requireNotNull(x)`/`checkNotNull(x)` when null is a programming error.

Common pitfalls

Sprinkling `!!` everywhere (you reintroduce NPE).
Assuming smart casts always work (they depend on mutability and visibility).

Related questions

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

#kotlin#lateinit#null-safety

Kotlin smart casts: when do they work and when do they not?

#kotlin#smart-cast#type-safety

What is `lateinit` and when can you use it?

#lateinit#null-safety