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Kotlin Sequences

Delay a transformation until a terminal operation needs its values.

How it works

asSequence creates a lazy pipeline. Intermediate operations describe work; a terminal operation such as toList actually consumes values. Laziness can avoid unnecessary intermediate work, but it does not automatically make a small calculation faster and can change when side effects occur.

Run this Kotlin example

These are local Kotlin/JVM programs. Install a JDK and the compiler using the official Kotlin compiler guide, or use an IDE with Kotlin support. Check java -version and kotlinc -version in a new terminal. The reviewed compiler is Kotlin 2.4.21 with a JVM 21 target, executed on Temurin JDK 25.0.5. Use JDK 21 or newer for this target. Android, Kotlin/JS and Kotlin/Native require separate platform projects and are not the tested environment here.

Save the complete source as sequences.kt in its own learning folder. The package line gives this example a namespace, not a network address. Bhylu has no Kotlin browser runner; do not use the JavaScript or Python editor. Apache, PHP and MySQL are not required.

Compile with kotlinc sequences.kt -jvm-target 21 -include-runtime -d lesson.jar, then run java -jar lesson.jar. Use a learning folder you own and keep generated JARs outside the website. These sources require no third-party application libraries.

Download the complete sequences.kt · Download all source files with hash checks

For the optional collection, save download-examples.mjs in a new learning folder and run node download-examples.mjs. A local preview uses node download-examples.mjs http://127.0.0.1:8000/examples/kotlin/. It refuses an existing bhylu-kotlin-examples folder and installs or executes no Kotlin code. Compile one ordinary lesson at a time by filename.

Example

sequences.kt

package bhylu.lessons.k_sequences

fun main() {
    var visits = 0
    val pipeline = (1..10).asSequence().map { visits++; it * 2 }.filter { it > 4 }
    println(visits)
    println(pipeline.take(2).toList())
    println(visits)
}

Expected result

0
[6, 8]
4

Nothing runs until toList. Producing the first two accepted values needs the first four source integers. This example uses a visible counter only to explain evaluation order; normal transformations should preferably remain pure.

For exact language and library contracts, consult the official Kotlin documentation. These fictional fixtures are learning programs, not a deployed hire service.

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