Collections vs sequences

Kotlin language Mid-level Kotlin 2.0+ API 24+ Reviewed 31 Oct 2026 By Best Latest Apps

Predict the output

How many times is map called?

fun main() {
    var calls = 0
    val first = (1..1_000).asSequence()
        .map { calls++; it * 2 }
        .first { it > 10 }
    println("$first $calls")
}

Options12 612 100011 62 1

Hint 1 of 1

Does map run for every element before first checks?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A 12 6

Correct. Elements go one at a time; map runs for 1 to 6, and 6*2 = 12 is the first above 10.

Why not the others?

B 12 1000. That's what a List chain would do: map everything first.
C 11 6. map doubles, so values are even.
D 2 1. 2 isn't greater than 10.

30-second answer

List operations like map and filter run eagerly: each step builds a full intermediate list. A Sequence runs lazily, element by element, through the whole chain, and stops as soon as a terminal operation like first or take has what it needs. Use sequences for long chains on large collections or when you stop early; lists are faster for small ones.

Practise saying it · 30 s. No recording, just a timer

Explain collections vs sequences.

Say your answer out loud, then check which of these points you covered:

Lists are eager with intermediate lists
Sequences are lazy, per element
Good for large chains and early exit
Lists are faster for small data

Deep dive

asSequence() converts; toList() ends the chain.

Sequences can be infinite with generateSequence.

Some operations, like sorted, still need all elements.

Flow is the asynchronous cousin of Sequence: also lazy, but suspending.

Common mistakes

Using sequences for tiny lists and adding overhead.
Forgetting the terminal operation, so nothing runs.
Assuming sorted on a sequence stays lazy.

Senior take & follow-ups

Give the element-by-element picture and the early-exit example.

Follow-up questions

Think of your answer first, then open the hint.

What's the relationship between Sequence and Flow?
Show hint
Both are lazy streams; Flow can suspend.
Does take(3) process the whole source?
Show hint
On a sequence, no; it stops after three items pass.

Suggest a fix if something here is wrong or out of date.