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

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).