Add java examples to getting started guide, with language switch (#22919)

* Java version of new Hello World
* Define paradox groups, use shorthand directives
* Make tutorial apply to both java and scala
* Use sbt-paradox-akka

akka-docs/src/test/java/jdocs/tutorial_1/ActorHierarchyExperiments.java

@@ -0,0 +1,147 @@
+package jdocs.tutorial_1;
+
+import akka.actor.AbstractActor;
+import akka.actor.AbstractActor.Receive;
+import akka.actor.ActorRef;
+import akka.actor.ActorSystem;
+import akka.actor.Props;
+import akka.testkit.javadsl.TestKit;
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.scalatest.junit.JUnitSuite;
+
+//#print-refs
+class PrintMyActorRefActor extends AbstractActor {
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+        .matchEquals("printit", p -> {
+          ActorRef secondRef = getContext().actorOf(Props.empty(), "second-actor");
+          System.out.println("Second: " + secondRef);
+        })
+        .build();
+  }
+}
+//#print-refs
+
+//#start-stop
+class StartStopActor1 extends AbstractActor {
+  @Override
+  public void preStart() {
+    System.out.println("first started");
+    getContext().actorOf(Props.create(StartStopActor2.class), "second");
+  }
+
+  @Override
+  public void postStop() {
+    System.out.println("first stopped");
+  }
+
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+        .matchEquals("stop", s -> {
+          getContext().stop(getSelf());
+        })
+        .build();
+  }
+}
+
+class StartStopActor2 extends AbstractActor {
+  @Override
+  public void preStart() {
+    System.out.println("second started");
+  }
+
+  @Override
+  public void postStop() {
+    System.out.println("second stopped");
+  }
+
+  // Actor.emptyBehavior is a useful placeholder when we don't
+  // want to handle any messages in the actor.
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+        .build();
+  }
+}
+//#start-stop
+
+//#supervise
+class SupervisingActor extends AbstractActor {
+  ActorRef child = getContext().actorOf(Props.create(SupervisedActor.class), "supervised-actor");
+
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+        .matchEquals("failChild", f -> {
+          child.tell("fail", getSelf());
+        })
+        .build();
+  }
+}
+
+class SupervisedActor extends AbstractActor {
+  @Override
+  public void preStart() {
+    System.out.println("supervised actor started");
+  }
+
+  @Override
+  public void postStop() {
+    System.out.println("supervised actor stopped");
+  }
+
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+        .matchEquals("fail", f -> {
+          System.out.println("supervised actor fails now");
+          throw new Exception("I failed!");
+        })
+        .build();
+  }
+}
+//#supervise
+
+class ActorHierarchyExperimentsTest extends JUnitSuite {
+  static ActorSystem system;
+
+  @BeforeClass
+  public static void setup() {
+    system = ActorSystem.create();
+  }
+
+  @AfterClass
+  public static void teardown() {
+    TestKit.shutdownActorSystem(system);
+    system = null;
+  }
+
+  @Test
+  public void testCreateTopAndChildActor() {
+    //#print-refs
+    ActorRef firstRef = system.actorOf(Props.create(PrintMyActorRefActor.class), "first-actor");
+    System.out.println("First : " + firstRef);
+    firstRef.tell("printit", ActorRef.noSender());
+    //#print-refs
+  }
+
+  @Test
+  public void testStartAndStopActors() {
+    //#start-stop
+    ActorRef first = system.actorOf(Props.create(StartStopActor1.class), "first");
+    first.tell("stop", ActorRef.noSender());
+    //#start-stop
+  }
+
+  @Test
+  public void testSuperviseActors() {
+    //#supervise
+    ActorRef supervisingActor = system.actorOf(Props.create(SupervisingActor.class), "supervising-actor");
+    supervisingActor.tell("failChild", ActorRef.noSender());
+    //#supervise
+  }
+}