Set Basics in Kotlin

Introduction

Set Basics is a fundamental concept every Kotlin developer should understand. Collections let you store and transform groups of values. Kotlin separates read-only and mutable views so you can express intent clearly in your APIs.

Set stores unique elements without guaranteed order (for hash sets). In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Set stores unique elements without guaranteed order (for hash sets).
• setOf() creates an immutable set.
• Duplicate values are automatically ignored.

Syntax

val set = setOf(1, 2, 2, 3)

Set Basics in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val ids = setOf(10, 20, 20, 30)
    println(ids)
    println("Size: ${ids.size}")
}

Sample Output

[10, 20, 30]
Size: 3

When to use

Use collections when the number of items is dynamic or when you need map/set semantics instead of a plain list.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. val ids = setOf(10, 20, 20, 30) assigns or updates a value used later in the program.

3. The println(ids) statement writes a line to the console — this produces part of the sample output below.

4. The println("Size: ${ids.size}") statement writes a line to the console — this produces part of the sample output below.

5. Set stores unique elements without guaranteed order (for hash sets).

6. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Prefer immutable listOf, setOf, mapOf for data that should not change after creation.
• Use map, filter, and fold instead of manual loops when transforming collections.
• Pick the smallest collection type that fits — don’t use a List when a Set is semantically correct.

Common Mistakes

• Modifying a list while iterating it — use filter or iterator remove carefully.
• Using mutableListOf when an immutable list would suffice, exposing accidental mutation.
• Calling get on a Map without checking key existence — prefer getOrDefault or getValue.

Key Points

• Set stores unique elements without guaranteed order (for hash sets).
• setOf() creates an immutable set.
• Duplicate values are automatically ignored.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.