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/Device.java

@@ -0,0 +1,103 @@
+/**
+ * Copyright (C) 2009-2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package jdocs.tutorial_4;
+
+import akka.actor.AbstractActor;
+import akka.actor.Props;
+import akka.event.Logging;
+import akka.event.LoggingAdapter;
+
+import jdocs.tutorial_4.DeviceManager.DeviceRegistered;
+import jdocs.tutorial_4.DeviceManager.RequestTrackDevice;
+
+import java.util.Optional;
+
+public class Device extends AbstractActor {
+  private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);
+
+  final String groupId;
+
+  final String deviceId;
+
+  public Device(String groupId, String deviceId) {
+    this.groupId = groupId;
+    this.deviceId = deviceId;
+  }
+
+  public static Props props(String groupId, String deviceId) {
+    return Props.create(Device.class, groupId, deviceId);
+  }
+
+  public static final class RecordTemperature {
+    final long requestId;
+    final double value;
+
+    public RecordTemperature(long requestId, double value) {
+      this.requestId = requestId;
+      this.value = value;
+    }
+  }
+
+  public static final class TemperatureRecorded {
+    final long requestId;
+
+    public TemperatureRecorded(long requestId) {
+      this.requestId = requestId;
+    }
+  }
+
+  public static final class ReadTemperature {
+    final long requestId;
+
+    public ReadTemperature(long requestId) {
+      this.requestId = requestId;
+    }
+  }
+
+  public static final class RespondTemperature {
+    final long requestId;
+    final Optional<Double> value;
+
+    public RespondTemperature(long requestId, Optional<Double> value) {
+      this.requestId = requestId;
+      this.value = value;
+    }
+  }
+
+  Optional<Double> lastTemperatureReading = Optional.empty();
+
+  @Override
+  public void preStart() {
+    log.info("Device actor {}-{} started", groupId, deviceId);
+  }
+
+  @Override
+  public void postStop() {
+    log.info("Device actor {}-{} stopped", groupId, deviceId);
+  }
+
+  @Override
+  public Receive createReceive() {
+    return receiveBuilder()
+            .match(RequestTrackDevice.class, r -> {
+              if (this.groupId.equals(r.groupId) && this.deviceId.equals(r.deviceId)) {
+                getSender().tell(new DeviceRegistered(), getSelf());
+              } else {
+                log.warning(
+                        "Ignoring TrackDevice request for {}-{}.This actor is responsible for {}-{}.",
+                        r.groupId, r.deviceId, this.groupId, this.deviceId
+                );
+              }
+            })
+            .match(RecordTemperature.class, r -> {
+              log.info("Recorded temperature reading {} with {}", r.value, r.requestId);
+              lastTemperatureReading = Optional.of(r.value);
+              getSender().tell(new TemperatureRecorded(r.requestId), getSelf());
+            })
+            .match(ReadTemperature.class, r -> {
+              getSender().tell(new RespondTemperature(r.requestId, lastTemperatureReading), getSelf());
+            })
+            .build();
+  }
+}