Android Academy · Lesson

Kotlin Collections

Work with List, Set, and Map. Use functional operators like filter, map, sortedBy, forEach, find, any, all, and chain operations for concise data transformations.

Kotlin Collections is a free Android Academy lesson on CoddyKit — lesson 1 of 6. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Android Academy learning path, one of 6 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Collections Overview

Kotlin's standard library provides three main collection types:

List — ordered sequence of elements (can have duplicates)
Set — unique elements, no specific order
Map — key-value pairs, keys are unique

Each type has an immutable version (read-only) and a mutable version (can be modified).

Lists: listOf vs mutableListOf

Create lists using factory functions:

fun main() {
    // Immutable — cannot add/remove items
    val fruits = listOf("Apple", "Banana", "Cherry")
    println(fruits[0])       // Apple
    println(fruits.size)     // 3

    // Mutable — can modify
    val scores = mutableListOf(10, 20, 30)
    scores.add(40)
    scores.removeAt(0)       // remove index 0
    println(scores)          // [20, 30, 40]
}

Map: Key-Value Pairs

Use mapOf and mutableMapOf to create maps. Access values by key:

fun main() {
    val capitals = mapOf(
        "Turkey" to "Ankara",
        "Germany" to "Berlin",
        "Japan" to "Tokyo"
    )

    println(capitals["Turkey"])      // Ankara
    println(capitals["France"])      // null (not found)
    println(capitals.getOrDefault("France", "Unknown")) // Unknown

    // Iterate
    for ((country, city) in capitals) {
        println("$country -> $city")
    }
}

filter — Select Items

filter returns a new list containing only elements that match a condition:

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5, 6, 7, 8)

    val evens = numbers.filter { it % 2 == 0 }
    println(evens)   // [2, 4, 6, 8]

    val names = listOf("Alice", "Bob", "Anna", "Charlie")
    val aNames = names.filter { it.startsWith("A") }
    println(aNames)  // [Alice, Anna]
}

map — Transform Items

map transforms every element and returns a new list:

fun main() {
    val prices = listOf(10.0, 20.0, 30.0)

    // Apply 10% tax
    val withTax = prices.map { it * 1.1 }
    println(withTax)  // [11.0, 22.0, 33.0]

    val names = listOf("alice", "bob", "carol")
    val upper = names.map { it.uppercase() }
    println(upper)    // [ALICE, BOB, CAROL]
}

sortedBy & sortedByDescending

Sort a list by a property:

data class Student(val name: String, val grade: Int)

fun main() {
    val students = listOf(
        Student("Alice", 88),
        Student("Bob", 95),
        Student("Carol", 72)
    )

    val byGrade = students.sortedByDescending { it.grade }
    for (s in byGrade) println("${s.name}: ${s.grade}")
    // Bob: 95
    // Alice: 88
    // Carol: 72
}

forEach & find

Two more useful collection functions:

forEach — iterate without building a result
find — return the first item matching a condition, or null

Chaining Operations

Chain multiple operations for concise data transformations:

data class Product(val name: String, val price: Double, val inStock: Boolean)

fun main() {
    val products = listOf(
        Product("Laptop", 999.0, true),
        Product("Mouse", 29.0, false),
        Product("Keyboard", 79.0, true),
        Product("Monitor", 349.0, true)
    )

    // In-stock products under $500, sorted by price
    val result = products
        .filter { it.inStock && it.price < 500 }
        .sortedBy { it.price }
        .map { "${it.name}: ${it.price}" }

    result.forEach { println(it) }
}

any, all, count

Aggregate functions for quick answers:

any { condition } — true if at least one item matches
all { condition } — true if every item matches
count { condition } — number of matching items
sumOf { it.price } — sum a numeric property

Quick Check

Which function would you use to get only the items in a list that satisfy a condition?

Recap: Kotlin Collections

You can now work with data in Kotlin efficiently:

listOf / mutableListOf, mapOf / mutableMapOf
filter — select matching items
map — transform items
sortedBy / sortedByDescending
Chain operations for clean, readable data pipelines

Next: display lists on screen with RecyclerView.

Frequently asked questions

Is the “Kotlin Collections” lesson free?

Yes — the full text of “Kotlin Collections” is free to read here on the web, and the Android Academy course includes 6 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Android Academy course, upgrade to CoddyKit PRO.

What will I learn in “Kotlin Collections”?

Work with List, Set, and Map. Use functional operators like filter, map, sortedBy, forEach, find, any, all, and chain operations for concise data transformations. You practise Android Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Android Academy?

No prior experience is required. Android Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 6, so you can start here or from the beginning and move at your own pace.

How long does the “Kotlin Collections” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Android Academy lesson?

Yes. Every Android Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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