Collection Iteration in Kotlin

Introduction

Collection Iteration 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.

Collections can be iterated using for loops. 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

• Collections can be iterated using for loops.
• forEach executes a lambda for each element.
• Destructuring can be used with maps.

Syntax

list.forEach { println(it) }

Collection Iteration in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val colors = listOf("Red", "Green", "Blue")
    colors.forEach { color ->
        println("Color: $color")
    }
}

Sample Output

Color: Red
Color: Green
Color: Blue

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 colors = listOf("Red", "Green", "Blue") assigns or updates a value used later in the program.

3. The println("Color: $color") statement writes a line to the console — this produces part of the sample output below.

4. Collections can be iterated using for loops.

5. 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

• Collections can be iterated using for loops.
• forEach executes a lambda for each element.
• Destructuring can be used with maps.
• 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.