Sequence vs List in Kotlin

Introduction

Sequence vs List is a fundamental concept every Kotlin developer should understand. Sequences evaluate lazily, computing each element only when needed. They shine on large datasets or chained transformations where eager lists would waste memory.

List operations are eager and create intermediate lists. 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

• List operations are eager and create intermediate lists.
• Sequence operations are lazy and chain processing.
• Choose sequence for large datasets and long chains.

Syntax

list.map{}.filter{}  // eager
seq.map{}.filter{} // lazy

Sequence vs List in Kotlin Example Program in Kotlin

fun main(args: Array<String>) {
    val listResult = listOf(1, 2, 3).map { it + 1 }.filter { it > 2 }
    val seqResult = sequenceOf(1, 2, 3).map { it + 1 }.filter { it > 2 }.toList()
    println("List: $listResult")
    println("Sequence: $seqResult")
}

Sample Output

List: [3, 4]
Sequence: [3, 4]

When to use

Use sequences when processing long chains of transformations on large collections and you want lazy evaluation.

How it works

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

2. val listResult = listOf(1, 2, 3).map { it + 1 }.filter { it > 2 } assigns or updates a value used later in the program.

3. val seqResult = sequenceOf(1, 2, 3).map { it + 1 }.filter { it > 2 }.toList() assigns or updates a value used later in the program.

4. The println("List: $listResult") statement writes a line to the console — this produces part of the sample output below.

5. The println("Sequence: $seqResult") statement writes a line to the console — this produces part of the sample output below.

6. List operations are eager and create intermediate lists.

7. 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

• Understand the core idea: list operations are eager and create intermediate lists.
• Prefer readable names and small functions so examples map directly to real projects.
• Run and modify the example — change values and observe how the output changes.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• List operations are eager and create intermediate lists.
• Sequence operations are lazy and chain processing.
• Choose sequence for large datasets and long chains.
• 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.