java futures / published by https://github.com/dacr/code-examples-manager #2f285c08-14f5-4309-af23-d156994901c9/431d2e4dfbffc8849e56f6ca1d37c1c7e99d8abe

java-futures.sc

// summary : java futures
// keywords : java, learning, futures, @testable
// publish : gist
// authors : David Crosson
// license : Apache NON-AI License Version 2.0 (https://raw.githubusercontent.com/non-ai-licenses/non-ai-licenses/main/NON-AI-APACHE2)
// id : 2f285c08-14f5-4309-af23-d156994901c9
// created-on : 2020-09-08T21:38:26+02:00
// managed-by : https://github.com/dacr/code-examples-manager
// run-with : scala-cli $file

// ---------------------
//> using scala  "3.3.0"
//> using dep "org.scalatest::scalatest:3.2.16"
//> using objectWrapper
// ---------------------

import org.scalatest._, flatspec._, matchers._, OptionValues._, concurrent._

import java.util.concurrent._
import java.util.function._
import scala.jdk.FutureConverters._
import scala.concurrent.{Future => ScalaFuture, Await}
import scala.concurrent.duration.Duration

object FutureTools {
  // convert a java future to a scala future as future converters only work with completable future...
  def javaFutureToScala[T](javaFuture: java.util.concurrent.Future[T]): scala.concurrent.Future[T] = {
    val completableFuture: CompletableFuture[T] = CompletableFuture.supplyAsync(new Supplier[T] {
      override def get(): T = javaFuture.get
    })
    completableFuture.asScala
  }

  // await infinitely for a java future to return its value
  def awaitJavaFutureResult[T](javaFuture: java.util.concurrent.Future[T]): T = { // of course never use this ;)
    Await.result(javaFutureToScala(javaFuture), Duration.Inf) // Of course never do this
  }
}


class JavaLanguageAsyncBasicsTest extends AsyncFlatSpec with should.Matchers with ScalaFutures {

  // ---------------------------------------------------------------------------------------
  "java 5 futures" should "allow future creation (old implementation)" in {
    val executor = Executors.newFixedThreadPool(4)
    val javaOldFuture: FutureTask[String] = new FutureTask[String](
      new Callable[String]() {
        override def call(): String = "It works"
      }
    )
    executor.execute(javaOldFuture)
    ScalaFuture(javaOldFuture.get).map(result => result shouldBe "It works")
  }
  // ---------------------------------------------------------------------------------------
  "java 8 futures" should "allow future creation (new implementation)" in {
    //val executor = Executors.newFixedThreadPool(4)
    val javaFuture = CompletableFuture.supplyAsync(new Supplier[String] {
      override def get(): String = "It works"
    })
    javaFuture.asScala.map { result => result shouldBe "It works" }
  }
  // ---------------------------------------------------------------------------------------
  it should "work with lambda expression" in {
    val javaFuture =
      CompletableFuture
        .supplyAsync(() => "It works")
    javaFuture.asScala.map { result => result shouldBe "It works" }
  }
  // ---------------------------------------------------------------------------------------
  it should "be possible to compose intermediary results (map)" in {
    val javaFuture =
      CompletableFuture
        .supplyAsync(() => "It works")
        .handleAsync((result, error) => result.toUpperCase) // map
    javaFuture.asScala.map { result => result shouldBe "IT WORKS" }
  }
  // ---------------------------------------------------------------------------------------
}


org.scalatest.tools.Runner.main(Array("-oDF", "-s", classOf[JavaLanguageAsyncBasicsTest].getName))