Kotlin

Null safety

• ? makes nullability explicit in the type — String? vs String.
• Safe-call ?. returns null instead of throwing. Elvis ?: provides a default.
• Never use !! in production code — it is a promise you will never break that can't be verified.

val length = name?.trim()?.length ?: 0
user?.email?.let { send(it) }   // null-guard + scoping

Data classes and sealed interfaces

• data class for value containers: auto-generates equals, hashCode, toString, copy, and destructuring.
• sealed interface closes a hierarchy and enables exhaustive when without an else branch.