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Extension functions add methods to existing types without modifying them. Defined outside the class:

fun String.shout(): String = this.uppercase() + "!"

"hello".shout()       // "HELLO!"

Inside the function body, this refers to the receiver. The function is dispatched statically based on the static type — it's not real polymorphism, just syntactic sugar.

Use cases:

• Add domain-specific methods to standard library types
• Make code read more like natural language: 5.minutes, "abc".isPalindrome()
• Provide null-safe variants: String?.orEmpty()

Extension on nullable types:

fun String?.isNullOrShort(): Boolean = this == null || this.length < 3
val s: String? = null
println(s.isNullOrShort())   // true (no NPE)

Scope-limited — extensions are not part of the type. They're resolved by import:

// In file utils.kt:
fun List<Int>.sumOfSquares(): Int = sumOf { it * it }

// In another file:
import com.example.utils.sumOfSquares
listOf(1, 2, 3).sumOfSquares()   // 14

Kotlin's standard library is built largely on extensions — List.map, String.toUpperCase(), etc. are all extensions on existing types.