CODEWITHSUNDEEP
javakotlinjava-stream
Evgeny Mamaev
Evgeny Mamaev

Reputation: 1357

Kotlin Stream peek(...) method

What is the best alternative in Kotlin to java.util.stream.Stream<>.peek(...)?

https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#peek-java.util.function.Consumer-

Seems there are no alternative intermediate operations:

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.streams/index.html

I found only terminating forEach(...)

Upvotes: 29

Views: 14690

Answers (3)

Andy Cass
Andy Cass

Reputation: 446

You can use your own custom extension function, I use this for live data

inline fun <T> LiveData<T>.peek(crossinline block: (T) -> Unit): LiveData<T> {
    return map {
        block(it)
        it
    }
}

You could modify to get something like this.

inline fun <T> Stream<T>.peek(crossinline block: (T) -> Unit): Stream<T> {
    return map {
        block(it)
        it
    }
}

Similarly

inline fun <T> Sequence<T>.peek(crossinline action: (T) -> Unit): Sequence<T> {
    return map {
        action(it)
        it
    }
}

Then in your code

listOf(1, 2, 3, 4, 5)
    .asSequence()
    .filter { it < 4 }
    .peek { println("peek $it") }
    .map { it * 10 }
    .peek { println("final: $it") }

Upvotes: 0

Mr.Q
Mr.Q

Reputation: 4524

Firstly, unlike Java in Kotlin, you can perform stream processing (map/reduce operations) on any type of collection for example:

val list = ArrayList<Int>()
list.forEach {  }
list.onEach {  }

However the operations defined in this way are not lazily evaluated and if we need lazy evaluation by applying the method .asSequence() and generate a stream from collection.

Finally to answer your question onEach() is the equivalent of peek()

Upvotes: 4

s1m0nw1
s1m0nw1

Reputation: 81879

The Stream alternative in Kotlin is Sequences.

 listOf(1, 2, 3, 4, 5)
    .asSequence()
    .filter { it < 3 }
    .onEach { println("filtered $it") }
    .map { it * 10 }
    .forEach { println("final: $it") }

There's onEach to do what peek does.

Fun fact: Kotlin also wanted to call their sequences "Streams" before it was clear that Java would do the same, so they renamed it to "Sequences".

Upvotes: 56

Related Questions