content.java

FoldDrain<String, String> fold = new FoldDrain<String, String>();
    Future<String> got = materialized.getDrainFor(fold);
  }
}

class FoldDrain<In, Res> extends DrainWithTheKey<In, Future<Res>> { }

class DrainWithTheKey<In, Res> {
  public Res something() { return null; }
}

class Materialized {
  public <T> T getDrainFor(DrainWithTheKey<?, T> drainKey) { return null; }
}