From Zero To Hero Series(Kotlin Sequences, Channels, Flows) — Part 1— Sequences

Kotlin language constains a container type which is called sequences. Sequences are very similar to lists. The difference between a list and sequence is that the sequences are lazily evaluated.

We can create sequences with different ways:

1 — From elements — By calling sequenceOf()

val numbersSequence = sequenceOf("four", "three", "two", "one")

We give elements as parameter to sequenceOf() function.

2- From iterable objects — By calling asSequence() function of an iterable object such as a List or Set.

val numbers = listOf("one", "two", "three", "four")
val numbersSequence = numbers.asSequence()

We can create a sequence by calling asSequence() function of an iterable object such as a List or Set.

3-From Chunks — sequence {}, yield, yieldAll()

val seqFromChunks = sequence {
    yield(1)
    yieldAll((2..5).toList())
    yield(6)
}

Sequences execute lazily.

• What does it mean ? Can you show me with an example ?
• Yeap. I will show you it in example 1.

Sequence Example 1 :

Compare the two examples above. First example is not using sequences. Second example is using sequences.

Sequence Example 2 :

In this example, we use sequence{} to create a sequence. When we call

sequence.take(5).toList()

the codes inside the sequence{} start to work. When 5 elements are taken into sequence, the execution stops. Thus the next lines of codes are not executed. Analyse the following examples to understand.

These two examples shows creating sequence from chunks. Lazy evaluation works here too.

Note that,

• sequences are not asynchoronous.
• sequences don’t support backpressure.

Benchmark Results between Sequences, RxJava and Flows

According to a benchmark results on scrabble game, Kotlin Flows are nearly 2 times faster than RxJava. Flows are a bit slower than Sequences. However Flows give you more possibilities, you can have asynchronous data producers, it supports backpressure. Since Flows support backpressure, it comes with a little overhead thus it’s a bit slower than sequences.

KotlinConf 2019: Asynchronous Data Streams with Kotlin Flow by Roman Elizarov

Difference between Sequence and Flow

Flows are asynchronous data stream.

A flow is an asynchronous version of a Sequence, a type of collection whose values are lazily produced. Just like a sequence, a flow produces each value on-demand whenever the value is needed, and flows can contain an infinite number of values.

In my article, i got help from the following links. Check these links for more information.

Lazy sequences in Kotlin | en.proft.me

Sequences | Kotlin

That’s all for now. See you in the 2nd part of this article for Channels.

Happy Sequence Coding.