Java example code for Getting Started

* WiP on Java version of new Hello World

Done:
- implementation for first 3 sections
- tests for first 2 sections

To do:
- formatting
- tests for section 3
- section 4 and 5
- hooking the fragments into the docs themselves

* Finish tutorial_03 conversion

* Converted tutorial 04

* Convert step 5 scala->java

* Reformat Java code (thx IntelliJ IDEA)

* Format using 2 spaces

Really it'd be neat to have something like scalariform for the Java code, too

* Make fields 'private final'

* Make message classes 'public static final'

* Use primitive long/double where possible

* sender() -> getSender()

* Static import of org.junit.Assert....

* Final actor fields

* println() the instruction to hit ENTER to quit

* Use jdocs package for quickstart, too

* Use helper methods for message handling

* Compare doubles specifying a delta

* Keep tutorial steps independent

akka-docs-new/src/test/java/jdocs/tutorial_4/DeviceManager.java

@@ -0,0 +1,80 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+
+package jdocs.tutorial_4;
+
+import akka.actor.AbstractActor;
+import akka.actor.ActorRef;
+import akka.actor.Props;
+import akka.actor.Terminated;
+import akka.event.Logging;
+import akka.event.LoggingAdapter;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class DeviceManager extends AbstractActor {
+  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);
+
+  public static Props props() {
+    return Props.create(DeviceManager.class);
+  }
+
+  public static final class RequestTrackDevice {
+    public final String groupId;
+    public final String deviceId;
+
+    public RequestTrackDevice(String groupId, String deviceId) {
+      this.groupId = groupId;
+      this.deviceId = deviceId;
+    }
+  }
+
+  public static final class DeviceRegistered {
+  }
+
+  final Map<String, ActorRef> groupIdToActor = new HashMap<>();
+  final Map<ActorRef, String> actorToGroupId = new HashMap<>();
+
+  @Override
+  public void preStart() {
+    log.info("DeviceManager started");
+  }
+
+  @Override
+  public void postStop() {
+    log.info("DeviceManager stopped");
+  }
+
+  private void onTrackDevice(RequestTrackDevice trackMsg) {
+    String groupId = trackMsg.groupId;
+    ActorRef ref = groupIdToActor.get(groupId);
+    if (ref != null) {
+      ref.forward(trackMsg, getContext());
+    } else {
+      log.info("Creating device group actor for {}", groupId);
+      ActorRef groupActor = getContext().actorOf(DeviceGroup.props(groupId), "group-" + groupId);
+      getContext().watch(groupActor);
+      groupActor.forward(trackMsg, getContext());
+      groupIdToActor.put(groupId, groupActor);
+      actorToGroupId.put(groupActor, groupId);
+    }
+  }
+
+  private void onTerminated(Terminated t) {
+    ActorRef groupActor = t.getActor();
+    String groupId = actorToGroupId.get(groupActor);
+    log.info("Device group actor for {} has been terminated", groupId);
+    actorToGroupId.remove(groupActor);
+    groupIdToActor.remove(groupId);
+  }
+
+  public Receive createReceive() {
+    return receiveBuilder()
+            .match(RequestTrackDevice.class, this::onTrackDevice)
+            .match(Terminated.class, this::onTerminated)
+            .build();
+  }
+
+}