collect

Apply a partial function to each incoming element, if the partial function is defined for a value the returned value is passed downstream.

Simple operators

Signature

Source.collectSource.collect Flow.collectFlow.collect

Description

Apply a partial function to each incoming element, if the partial function is defined for a value the returned value is passed downstream. This can often replace filter followed by map to achieve the same in one single operator.

collect is supposed to be used with PFBuilderPFBuilder to construct the partial function. There is also a collectType that often can be easier to use than the PFBuilder and then combine with ordinary filter and map operators.

Example

Given stream element classes Message, Ping, and Pong, where Ping extends Message and Pong is an unrelated class.

Scala

sourcetrait Message
final case class Ping(id: Int) extends Message
final case class Pong(id: Int)

Java

sourcestatic interface Message {}

static class Ping implements Message {
  final int id;

  Ping(int id) {
    this.id = id;
  }
}

static class Pong {
  final int id;

  Pong(int id) {
    this.id = id;
  }
}

From a stream of Message elements we would like to collect all elements of type Ping that have an id != 0, and then covert to Pong with same id.

Scala

sourceval flow: Flow[Message, Pong, NotUsed] =
  Flow[Message].collect {
    case Ping(id) if id != 0 => Pong(id)
  }

Java

sourceFlow<Message, Pong, NotUsed> flow =
    Flow.of(Message.class)
        .collect(
            new PFBuilder<Message, Pong>()
                .match(Ping.class, p -> p.id != 0, p -> new Pong(p.id))
                .build());

An alternative is to use collectType. The same conversion be written as follows, and it is as efficient.

Java

sourceFlow<Message, Pong, NotUsed> flow =
    Flow.of(Message.class)
        .collectType(Ping.class)
        .filter(p -> p.id != 0)
        .map(p -> new Pong(p.id));

Reactive Streams semantics

emits when the provided partial function is defined for the element

backpressures the partial function is defined for the element and downstream backpressures

completes when upstream completes