Kotlin Collections – Lists, Sets, and Maps - Learn ANDROID-KOTLIN | Revochamp

Name: Kotlin Collections – Lists, Sets, and Maps
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Author: Revochamp

Immutability by Default

In Kotlin, the collections library is split into two hierarchies: Read-Only (covariant) and Mutable. As an Engineering Manager, you should default to read-only collections to prevent 'side-effect' bugs in your native Android modules. This aligns with the 'unidirectional data flow' patterns used in modern Flutter architectures.

Lists: Ordered Collections

A List stores elements in a specific order and allows duplicates. Use listOf() for a fixed list and mutableListOf() if you need to add or remove elements at runtime.

KOTLINRead-only

// Read-only list
val tags = listOf("AI", "Mobile", "Flutter")
// tags.add("Dart") // Compilation Error

// Mutable list
val widgets = mutableListOf("Button", "Icon")
widgets.add("Switch") // OK

// Read-only list
val tags = listOf("AI", "Mobile", "Flutter")
// tags.add("Dart") // Compilation Error

// Mutable list
val widgets = mutableListOf("Button", "Icon")
widgets.add("Switch") // OK

Sets and Maps

Sets store unique elements, while Maps store key-value pairs. Kotlin's mapOf uses the to keyword to create a readable 'Pair' syntax, making configuration objects for your Revochamp engine easy to define.

KOTLINRead-only

// Set (Unique items only)
val uniqueIds = setOf("id_1", "id_2", "id_1") // Contains 2 items

// Map (Key-Value pairs)
val config = mapOf(
    "theme" to "dark",
    "version" to "2.0",
    "isEnabled" to true
)

println(config["theme"]) // "dark"

// Set (Unique items only)
val uniqueIds = setOf("id_1", "id_2", "id_1") // Contains 2 items

// Map (Key-Value pairs)
val config = mapOf(
    "theme" to "dark",
    "version" to "2.0",
    "isEnabled" to true
)

println(config["theme"]) // "dark"

Functional Operators

Kotlin provides a rich set of higher-order functions for processing collections. These are highly optimized and allow you to transform data from your Python API into native Android models without using complex loops.

KOTLINRead-only

val numbers = listOf(1, 2, 3, 4, 5, 6)

// Filter: Keep only even numbers
val evens = numbers.filter { it % 2 == 0 }

// Map: Transform each number
val squared = numbers.map { it * it }

// Find: Locate the first element matching a condition
val firstBig = numbers.find { it > 4 } // Returns 5

val numbers = listOf(1, 2, 3, 4, 5, 6)

// Filter: Keep only even numbers
val evens = numbers.filter { it % 2 == 0 }

// Map: Transform each number
val squared = numbers.map { it * it }

// Find: Locate the first element matching a condition
val firstBig = numbers.find { it > 4 } // Returns 5

Collection Comparison: Kotlin vs. Dart

Feature	Kotlin	Dart (Flutter)
Read-only List	List (via listOf)	List (via const or Unmodifiable)
Mutable List	MutableList	List
Map Syntax	key to value	key: value
Filtering	.filter { }	.where((e) => )
Mapping	.map { }	.map((e) => ).toList()
First Match	.firstOrNull { }	.firstWhere((e) => , orElse: )

Test Your Knowledge

of 3

Which function is used to create a list that can be modified after initialization?

listOf()

mutableListOf()

fixedListOf()

array()

of 3

Which operator would you use to transform every element in a list into something else?

filter

find

map

reduce

of 3

What is the result of setOf(1, 2, 2, 3).size?

Error

Frequently Asked Questions

What is the difference between List and ArrayList in Kotlin?

In Kotlin, 'List' is an interface representing a read-only list. 'ArrayList' is a specific implementation of a MutableList based on a dynamic array. In native Android development, you usually use the factory methods 'listOf()' or 'mutableListOf()' rather than instantiating ArrayList directly.

How do I handle nulls in collections?

Kotlin provides 'filterNotNull()' which efficiently removes all null elements from a collection, and 'listOfNotNull()' which creates a list excluding any null arguments passed to it. This is extremely useful when parsing messy AI data.

What is a Sequence in Kotlin?

A Sequence is the Kotlin equivalent of Dart's 'Iterable'. Unlike a List, which performs transformations eagerly (creating a new list at each step), a Sequence performs them lazily. Use Sequences for large datasets to improve performance and reduce memory overhead.

// Set (Unique items only) val uniqueIds = setOf("id_1", "id_2", "id_1") // Contains 2 items // Map (Key-Value pairs) val config = mapOf( "theme" to "dark", "version" to "2.0", "isEnabled" to true ) println(config["theme"]) // "dark"

val numbers = listOf(1, 2, 3, 4, 5, 6) // Filter: Keep only even numbers val evens = numbers.filter { it % 2 == 0 } // Map: Transform each number val squared = numbers.map { it * it } // Find: Locate the first element matching a condition val firstBig = numbers.find { it > 4 } // Returns 5

Feature

Kotlin

Dart (Flutter)

Read-only List

List (via listOf)

List (via const or Unmodifiable)

Mutable List

MutableList

List

Map Syntax

key to value

key: value

Filtering

.filter { }

.where((e) => )

Mapping

.map { }

.map((e) => ).toList()

First Match

.firstOrNull { }

.firstWhere((e) => , orElse: )

Test Your Knowledge

of 3

Which function is used to create a list that can be modified after initialization?

listOf()

mutableListOf()

fixedListOf()

array()

of 3

Which operator would you use to transform every element in a list into something else?

filter

find

map

reduce

of 3

What is the result of setOf(1, 2, 2, 3).size?

Error

Frequently Asked Questions

What is the difference between List and ArrayList in Kotlin?

How do I handle nulls in collections?

What is a Sequence in Kotlin?

Kotlin Collections – Lists, Sets, and Maps

Kotlin Collections – Lists, Sets, and Maps

Immutability by Default

Lists: Ordered Collections

Sets and Maps

Functional Operators

Collection Comparison: Kotlin vs. Dart

Test Your Knowledge

Frequently Asked Questions

Need help?

Kotlin Collections – Lists, Sets, and Maps

Kotlin Collections – Lists, Sets, and Maps

Immutability by Default

Lists: Ordered Collections

Sets and Maps

Functional Operators

Collection Comparison: Kotlin vs. Dart

Test Your Knowledge

Frequently Asked Questions

Need help?