Kotlin Collections — Complete Guide

1. What Are Collections in Kotlin?

In Kotlin, a Collection is a data structure that holds multiple elements. Collections help you store, organize, and manipulate groups of related data efficiently.

Kotlin collections are categorized into three main types: Lists, Sets, and Maps. Collections can also be either read-only (immutable) or mutable.

2. Collection Hierarchy in Kotlin

• Collection<T> — Base interface for read-only collections.
• List<T> — Ordered collection with elements accessible by index.
• Set<T> — Collection of unique elements; order may or may not be preserved.
• Map<K, V> — Collection of key-value pairs.

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3. Types of Collections

1️⃣ Lists

A List is an ordered collection. Duplicates are allowed. Kotlin provides:

• List — Immutable (read-only)
• MutableList — Mutable (can add, remove, or update elements)

// Immutable List
val fruits = listOf("Apple", "Banana", "Cherry")

// Mutable List
val numbers = mutableListOf(1, 2, 3)
numbers.add(4)
        

2️⃣ Sets

A Set stores unique elements. Duplicates are automatically removed.

• Set — Immutable
• MutableSet — Mutable
• LinkedHashSet — Maintains insertion order
• HashSet — No guaranteed order

val uniqueNumbers = setOf(1, 2, 2, 3) // [1, 2, 3]
val mutableSet = mutableSetOf("A", "B")
mutableSet.add("C")
        

3️⃣ Maps

A Map stores key-value pairs. Keys are unique, values can repeat. Kotlin provides:

• Map<K, V> — Immutable
• MutableMap<K, V> — Mutable

// Immutable Map
val capitals = mapOf("Pakistan" to "Islamabad", "USA" to "Washington")

// Mutable Map
val mutableCapitals = mutableMapOf("India" to "New Delhi")
mutableCapitals["UK"] = "London"
        

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4. Functional Operations on Collections

Kotlin collections provide powerful higher-order functions for traversing, filtering, mapping, and more.

// Example: List operations
val numbers = listOf(1, 2, 3, 4, 5)

// Filter even numbers
val evenNumbers = numbers.filter { it % 2 == 0 }

// Map to double
val doubled = numbers.map { it * 2 }

// Find first number > 3
val firstLarge = numbers.find { it > 3 }

println(evenNumbers) // [2, 4]
println(doubled)     // [2, 4, 6, 8, 10]
println(firstLarge)  // 4
        

5. Traversing Collections

// Using for loop
for (num in numbers) println(num)

// Using forEach
numbers.forEach { println(it) }

// Traversing Sets
val namesSet = setOf("Ali", "Ahmed", "Zain")
namesSet.forEach { println(it) }

// Traversing Maps
val capitalsMap = mapOf("Pakistan" to "Islamabad", "USA" to "Washington")
capitalsMap.forEach { (country, capital) ->
    println("$country -> $capital")
}
        

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6. Key Differences Between Collection Types

Feature	List	Set	Map
Duplicates	Allowed	Not allowed	Keys unique, values can repeat
Order	Maintained	Not guaranteed (LinkedHashSet preserves)	Not guaranteed (LinkedHashMap preserves)
Index Access	Yes	No	No
Mutable / Immutable	List / MutableList	Set / MutableSet	Map / MutableMap
Functional Operations	filter, map, find, sorted...	filter, map, find...	mapKeys, mapValues, filterKeys...

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7. When to Use Which Collection?

• ✅ List: Ordered data, duplicates allowed, need index access (RecyclerView, API data).
• ✅ Set: Unique elements, no duplicates, perform set operations (permissions, tags).
• ✅ Map: Key-value pairs, fast lookups by key (settings, configuration, dictionaries).