A lambda is a function without a name that you can pass around as a value:

val square: (Int) -> Int = { x -> x * x }
println(square(5))   // 25

A higher-order function is one that takes a function as a parameter or returns one. You’ve already used these constantly — setOnClickListener { ... } is a higher-order function call.

fun List<Int>.sumIf(predicate: (Int) -> Boolean): Int {
    var total = 0
    for (item in this) {
        if (predicate(item)) total += item
    }
    return total
}

val numbers = listOf(1, 2, 3, 4, 5, 6)
val evenSum = numbers.sumIf { it % 2 == 0 }   // 12

That’s also an extension function — sumIf is added onto List<Int> without modifying the original class. Extension functions are how Kotlin adds conveniences to existing Java/Android classes (like String or View) without inheritance.

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