Kotlin: Collections & Lambdas

What you will learn

Kotlin's powerful functional collection operations, the distinction between read-only and mutable collections, and the implicit lambda parameter it.

Creating collections

val numbers = listOf(1, 2, 3, 4, 5)          // read-only List<Int>
val mutableNumbers = mutableListOf(1, 2, 3)  // MutableList<Int>
val set = setOf("a", "b", "c")            // read-only Set
val map = mapOf("key" to "value")          // read-only Map

Kotlin separates read-only and mutable at the type level. listOf() returns List<T> (read-only); mutableListOf() returns MutableList<T>.

Lambda operations

val even = numbers.filter { it % 2 == 0 }      // [2, 4]
val doubled = numbers.map { it * 2 }            // [2, 4, 6, 8, 10]
val sum = numbers.reduce { acc, n -> acc + n }  // 15
val anyBig = numbers.any { it > 4 }              // true
val sortedDesc = numbers.sortedDescending()      // [5, 4, 3, 2, 1]

The it keyword is the implicit parameter name when a lambda has exactly one parameter. For multiple parameters, name them explicitly: { acc, n -> ... }.

Sequence (lazy evaluation)

For large collections, use asSequence() to avoid creating intermediate lists:

numbers.asSequence()
    .filter { it > 2 }
    .map { it * 2 }
    .toList()

Common mistakes

• Calling .add() on a listOf() result — it's read-only! Use mutableListOf() or += which creates a new list.
• Forgetting that map { ... } on a Map iterates over entries, not keys — use .keys or .values if that's what you need.
• Chaining many operations on large lists without .asSequence() — each step creates a new list.

Quick check below!