content.java

/*
  trait MaterializedMap {
    def materializedDrain[Res](key: DrainWithKey[_, Res]): Res
  }
*/


class X {
  {
    MaterializedMap materializedMap = new MaterializedMap();
    RealFold<String, String> fold = new RealFold<String, String>();
    materializedMap.materializedDrain(fold);
  }
}
// extends the "real DrainwithKey" but with changed type member to type param:

class RealFold<In, U> implements DrainWithKey<In, Future<U>> {

  @Override public Future<U> runWith(TapWithKey<In> tap, FlowMaterializer materializer) {
    return null;
  }

  @Override
  public Future<U> attach(Publisher<In> flowPublisher, ActorBasedFlowMaterializer materializer, String flowName) {
    return null;
  }

  @Override public Tuple2<Subscriber<In>, Future<U>> create(ActorBasedFlowMaterializer materializer, String flowName) {
    return null;
  }

  @Override public boolean isActive() {
    return false;
  }
}

DrainWithKey.scala

trait DrainWithKey[-In, +MaterializedType] extends Drain[In, MaterializedType] {

//  type MaterializedType

  /**
   * Attach this drain to the given [[org.reactivestreams.Publisher]]. Using the given
   * [[FlowMaterializer]] is completely optional, especially if this drain belongs to
   * a different Reactive Streams implementation. It is the responsibility of the
   * caller to provide a suitable FlowMaterializer that can be used for running
   * Flows if necessary.
   *
   * @param flowPublisher the Publisher to consume elements from
   * @param materializer a FlowMaterializer that may be used for creating flows
   * @param flowName the name of the current flow, which should be used in log statements or error messages
   */
  def attach(flowPublisher: Publisher[In @uncheckedVariance], materializer: ActorBasedFlowMaterializer, flowName: String): MaterializedType
  /**
   * This method is only used for Drains that return true from [[#isActive]], which then must
   * implement it.
   */
  def create(materializer: ActorBasedFlowMaterializer, flowName: String): (Subscriber[In] @uncheckedVariance, MaterializedType) =
    throw new UnsupportedOperationException(s"forgot to implement create() for $getClass that says isActive==true")