Careeroza

Sequences vs Collections

Eager vs Lazy

Collection operations (list.map { }.filter { }) are eager: each step processes the whole list and creates a new list. A Sequence is lazy: each element goes through the whole chain one by one, and nothing runs until a terminal operation (toList, first, sum...).

List:      [1,2,3,4] ─map─► [..4 items..] ─filter─► [..] ─first─► result   (2 temporary lists)
Sequence:  1 ─map─filter─✗   2 ─map─filter─✓ first ─► result               (stops early, no lists)

See the Order

Example

fun main() {
    val nums = listOf(1, 2, 3, 4, 5)

    println("List (eager):")
    val a = nums.map { println("  map $it"); it * 10 }
        .filter { println("  filter $it"); it > 20 }
        .first()
    println("  result $a")

    println("Sequence (lazy):")
    val b = nums.asSequence()
        .map { println("  map $it"); it * 10 }
        .filter { println("  filter $it"); it > 20 }
        .first()
    println("  result $b")
}

Output

List (eager):
  map 1
  map 2
  map 3
  map 4
  map 5
  filter 10
  filter 20
  filter 30
  filter 40
  filter 50
  result 30
Sequence (lazy):
  map 1
  filter 10
  map 2
  filter 20
  map 3
  filter 30
  result 30

Comparison

	Iterable / List	Sequence
Evaluation	eager, step by step	lazy, element by element
Intermediate collections	one per operation	none
Can be infinite	no	yes
Fast for	small lists, few operations, need indexes	big data, many chained operations, `first`/`take`
Java equivalent	-	`java.util.stream.Stream` (sequential)