StreamConverters.javaCollector

Create a sink which materializes into a Future CompletionStage which will be completed with a result of the Java Collector transformation and reduction operations.

Additional Sink and Source converters

Signature

StreamConverters.javaCollectorStreamConverters.javaCollector

Description

javaCollector creates a SinkSink that accepts a Java 8 java.util.stream.Collector factory. The Collector accumulates incoming elements into a mutable container as downstream demand is triggered, and after the stream completes, an optional finisher transforms the accumulated result. The sink materializes into a FutureCompletionStage holding the final result. All processing happens sequentially on the stream’s materialized execution context.

Since each materialization of the sink must start with a fresh accumulator, the factory is invoked once per materialization. Use javaCollectorParallelUnordered if parallel collection is needed.

Sink.collect provides a simpler API for common collection scenarios.

Example

In this example, StreamConverters.javaCollector uses Collectors.toList to gather stream elements into a List.

Scala

copysourceval source: Source[String, NotUsed] =
  Source(List("Apache", "Pekko", "Streams"))

val sink: Sink[String, _] =
  StreamConverters.javaCollector(() => Collectors.toList[String]())

Java

copysourceSource<String, NotUsed> source = Source.from(Arrays.asList("Apache", "Pekko", "Streams"));

CompletionStage<List<String>> result =
    source.runWith(StreamConverters.javaCollector(Collectors::toList), system);