Merge pull request #36 from informatik-ag-ngl/f/message_notification

Implemented event system and status tray icon

.settings/org.eclipse.jdt.core.prefs

@@ -1,8 +1,107 @@
 eclipse.preferences.version=1
+org.eclipse.jdt.core.compiler.annotation.inheritNullAnnotations=disabled
+org.eclipse.jdt.core.compiler.annotation.missingNonNullByDefaultAnnotation=ignore
+org.eclipse.jdt.core.compiler.annotation.nonnull=org.eclipse.jdt.annotation.NonNull
+org.eclipse.jdt.core.compiler.annotation.nonnull.secondary=
+org.eclipse.jdt.core.compiler.annotation.nonnullbydefault=org.eclipse.jdt.annotation.NonNullByDefault
+org.eclipse.jdt.core.compiler.annotation.nonnullbydefault.secondary=
+org.eclipse.jdt.core.compiler.annotation.nullable=org.eclipse.jdt.annotation.Nullable
+org.eclipse.jdt.core.compiler.annotation.nullable.secondary=
+org.eclipse.jdt.core.compiler.annotation.nullanalysis=disabled
 org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.8
 org.eclipse.jdt.core.compiler.compliance=1.8
+org.eclipse.jdt.core.compiler.problem.APILeak=warning
+org.eclipse.jdt.core.compiler.problem.annotationSuperInterface=warning
+org.eclipse.jdt.core.compiler.problem.autoboxing=ignore
+org.eclipse.jdt.core.compiler.problem.comparingIdentical=warning
+org.eclipse.jdt.core.compiler.problem.deadCode=warning
+org.eclipse.jdt.core.compiler.problem.deprecation=warning
+org.eclipse.jdt.core.compiler.problem.deprecationInDeprecatedCode=disabled
+org.eclipse.jdt.core.compiler.problem.deprecationWhenOverridingDeprecatedMethod=disabled
+org.eclipse.jdt.core.compiler.problem.discouragedReference=warning
+org.eclipse.jdt.core.compiler.problem.emptyStatement=ignore
 org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=disabled
+org.eclipse.jdt.core.compiler.problem.explicitlyClosedAutoCloseable=ignore
+org.eclipse.jdt.core.compiler.problem.fallthroughCase=ignore
+org.eclipse.jdt.core.compiler.problem.fatalOptionalError=disabled
+org.eclipse.jdt.core.compiler.problem.fieldHiding=ignore
+org.eclipse.jdt.core.compiler.problem.finalParameterBound=warning
+org.eclipse.jdt.core.compiler.problem.finallyBlockNotCompletingNormally=warning
 org.eclipse.jdt.core.compiler.problem.forbiddenReference=warning
+org.eclipse.jdt.core.compiler.problem.hiddenCatchBlock=warning
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+org.eclipse.jdt.core.compiler.problem.incompatibleNonInheritedInterfaceMethod=warning
+org.eclipse.jdt.core.compiler.problem.incompleteEnumSwitch=ignore
+org.eclipse.jdt.core.compiler.problem.indirectStaticAccess=ignore
+org.eclipse.jdt.core.compiler.problem.localVariableHiding=ignore
+org.eclipse.jdt.core.compiler.problem.methodWithConstructorName=warning
+org.eclipse.jdt.core.compiler.problem.missingDefaultCase=ignore
+org.eclipse.jdt.core.compiler.problem.missingDeprecatedAnnotation=ignore
+org.eclipse.jdt.core.compiler.problem.missingEnumCaseDespiteDefault=disabled
+org.eclipse.jdt.core.compiler.problem.missingHashCodeMethod=ignore
+org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotation=ignore
+org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotationForInterfaceMethodImplementation=enabled
+org.eclipse.jdt.core.compiler.problem.missingSerialVersion=warning
+org.eclipse.jdt.core.compiler.problem.missingSynchronizedOnInheritedMethod=ignore
+org.eclipse.jdt.core.compiler.problem.noEffectAssignment=warning
+org.eclipse.jdt.core.compiler.problem.noImplicitStringConversion=warning
+org.eclipse.jdt.core.compiler.problem.nonExternalizedStringLiteral=ignore
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+org.eclipse.jdt.core.compiler.problem.nonnullTypeVariableFromLegacyInvocation=warning
+org.eclipse.jdt.core.compiler.problem.nullAnnotationInferenceConflict=error
+org.eclipse.jdt.core.compiler.problem.nullReference=warning
+org.eclipse.jdt.core.compiler.problem.nullSpecViolation=error
+org.eclipse.jdt.core.compiler.problem.nullUncheckedConversion=warning
+org.eclipse.jdt.core.compiler.problem.overridingPackageDefaultMethod=warning
+org.eclipse.jdt.core.compiler.problem.parameterAssignment=ignore
+org.eclipse.jdt.core.compiler.problem.pessimisticNullAnalysisForFreeTypeVariables=warning
+org.eclipse.jdt.core.compiler.problem.possibleAccidentalBooleanAssignment=ignore
+org.eclipse.jdt.core.compiler.problem.potentialNullReference=ignore
+org.eclipse.jdt.core.compiler.problem.potentiallyUnclosedCloseable=ignore
+org.eclipse.jdt.core.compiler.problem.rawTypeReference=warning
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+org.eclipse.jdt.core.compiler.problem.redundantNullCheck=ignore
+org.eclipse.jdt.core.compiler.problem.redundantSpecificationOfTypeArguments=ignore
+org.eclipse.jdt.core.compiler.problem.redundantSuperinterface=ignore
+org.eclipse.jdt.core.compiler.problem.reportMethodCanBePotentiallyStatic=ignore
+org.eclipse.jdt.core.compiler.problem.reportMethodCanBeStatic=ignore
 org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=ignore
+org.eclipse.jdt.core.compiler.problem.specialParameterHidingField=disabled
+org.eclipse.jdt.core.compiler.problem.staticAccessReceiver=warning
+org.eclipse.jdt.core.compiler.problem.suppressOptionalErrors=disabled
+org.eclipse.jdt.core.compiler.problem.suppressWarnings=enabled
+org.eclipse.jdt.core.compiler.problem.syntacticNullAnalysisForFields=disabled
+org.eclipse.jdt.core.compiler.problem.syntheticAccessEmulation=ignore
+org.eclipse.jdt.core.compiler.problem.terminalDeprecation=warning
+org.eclipse.jdt.core.compiler.problem.typeParameterHiding=warning
+org.eclipse.jdt.core.compiler.problem.unavoidableGenericTypeProblems=enabled
+org.eclipse.jdt.core.compiler.problem.uncheckedTypeOperation=warning
+org.eclipse.jdt.core.compiler.problem.unclosedCloseable=warning
+org.eclipse.jdt.core.compiler.problem.undocumentedEmptyBlock=ignore
+org.eclipse.jdt.core.compiler.problem.unhandledWarningToken=warning
+org.eclipse.jdt.core.compiler.problem.unlikelyCollectionMethodArgumentType=warning
+org.eclipse.jdt.core.compiler.problem.unlikelyCollectionMethodArgumentTypeStrict=disabled
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+org.eclipse.jdt.core.compiler.problem.unnecessaryElse=ignore
+org.eclipse.jdt.core.compiler.problem.unnecessaryTypeCheck=ignore
+org.eclipse.jdt.core.compiler.problem.unqualifiedFieldAccess=ignore
+org.eclipse.jdt.core.compiler.problem.unstableAutoModuleName=warning
+org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownException=ignore
+org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionExemptExceptionAndThrowable=enabled
+org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionIncludeDocCommentReference=enabled
+org.eclipse.jdt.core.compiler.problem.unusedDeclaredThrownExceptionWhenOverriding=disabled
+org.eclipse.jdt.core.compiler.problem.unusedExceptionParameter=ignore
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+org.eclipse.jdt.core.compiler.problem.unusedParameter=ignore
+org.eclipse.jdt.core.compiler.problem.unusedParameterIncludeDocCommentReference=enabled
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+org.eclipse.jdt.core.compiler.problem.unusedPrivateMember=warning
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 org.eclipse.jdt.core.compiler.release=disabled
 org.eclipse.jdt.core.compiler.source=1.8

src/main/java/envoy/client/LocalDB.java

@@ -14,6 +14,8 @@ import java.util.logging.Logger;
 import javax.xml.datatype.DatatypeConfigurationException;
 import javax.xml.datatype.DatatypeFactory;
 
+import envoy.client.event.EventBus;
+import envoy.client.event.MessageCreationEvent;
 import envoy.exception.EnvoyException;
 import envoy.schema.Message;
 import envoy.schema.Message.Metadata.MessageState;
@@ -38,12 +40,12 @@ public class LocalDB {
 	private ObjectFactory	objectFactory	= new ObjectFactory();
 	private DatatypeFactory	datatypeFactory;
 
-	private static final Logger logger = Logger.getLogger(LocalDB.class.getSimpleName());
-	
 	private Sync	unreadMessagesSync	= objectFactory.createSync();
 	private Sync	sync				= objectFactory.createSync();
 	private Sync	readMessages		= objectFactory.createSync();
 
+	private static final Logger logger = Logger.getLogger(LocalDB.class.getSimpleName());
+
 	/**
 	 * Constructs an empty local database.
 	 *
@@ -138,6 +140,13 @@ public class LocalDB {
 		return message;
 	}
 
+	/**
+	 * Creates a {@link Sync} object filled with the changes that occurred to the
+	 * local database since the last synchronization.
+	 * 
+	 * @param userId the ID of the user that is synchronized by this client
+	 * @return {@link Sync} object filled with the current changes
+	 */
 	public Sync fillSync(long userId) {
 		addWaitingMessagesToSync();
 
@@ -148,85 +157,72 @@ public class LocalDB {
 		return sync;
 	}
 
+	/**
+	 * Applies the changes carried by a {@link Sync} object to the local database
+	 * 
+	 * @param returnSync the {@link Sync} object to apply
+	 */
 	public void applySync(Sync returnSync) {
 		for (int i = 0; i < returnSync.getMessages().size(); i++) {
-			if (returnSync.getMessages().get(i).getMetadata().getMessageId() != 0
-					&& returnSync.getMessages().get(i).getMetadata().getState() == MessageState.SENT) {
-				// Update Local Messages with State WAITING (add Message ID and change State to
-				// SENT)
-				for (int j = 0; j < sync.getMessages().size(); j++) {
-					if (j == i) {
-						sync.getMessages().get(j).getMetadata().setMessageId(returnSync.getMessages().get(j).getMetadata().getMessageId());
-						sync.getMessages().get(j).getMetadata().setState(returnSync.getMessages().get(j).getMetadata().getState());
-					}
-				}
-			}
-			if (returnSync.getMessages().get(i).getMetadata().getMessageId() != 0 && returnSync.getMessages().get(i).getMetadata().getSender() != 0
-					&& returnSync.getMessages().get(i).getMetadata().getState() == MessageState.RECEIVED) {
+
+			// The message has an ID
+			if (returnSync.getMessages().get(i).getMetadata().getMessageId() != 0) {
+
+				// Messages are processes differently corresponding to their state
+				switch (returnSync.getMessages().get(i).getMetadata().getState()) {
+					case SENT:
+						// Update previously waiting and now sent messages that were assigned an ID by
+						// the server
+						sync.getMessages().get(i).getMetadata().setMessageId(returnSync.getMessages().get(i).getMetadata().getMessageId());
+						sync.getMessages().get(i).getMetadata().setState(returnSync.getMessages().get(i).getMetadata().getState());
+						break;
+					case RECEIVED:
+						if (returnSync.getMessages().get(i).getMetadata().getSender() != 0) {
 							// these are the unread Messages from the server
 							unreadMessagesSync.getMessages().add(returnSync.getMessages().get(i));
-			}
 
-			if (returnSync.getMessages().get(i).getMetadata().getMessageId() != 0 && returnSync.getMessages().get(i).getMetadata().getSender() == 0
-					&& returnSync.getMessages().get(i).getMetadata().getState() == MessageState.RECEIVED) {
+							// Create and dispatch message creation event
+							EventBus.getInstance().dispatch(new MessageCreationEvent(returnSync.getMessages().get(i)));
+						} else {
 							// Update Messages in localDB to state RECEIVED
-				for (int j = 0; j < getChats().size(); j++) {
-					if (getChats().get(j).getRecipient().getID() == returnSync.getMessages().get(i).getMetadata().getRecipient()) {
-						for (int k = 0; k < getChats().get(j).getModel().getSize(); k++) {
-							if (getChats().get(j).getModel().get(k).getMetadata().getMessageId() == returnSync.getMessages()
+							for (Chat chat : getChats())
+								if (chat.getRecipient().getID() == returnSync.getMessages().get(i).getMetadata().getRecipient())
+									for (int j = 0; j < chat.getModel().getSize(); j++)
+									if (chat.getModel().get(j).getMetadata().getMessageId() == returnSync.getMessages()
 										.get(i)
 										.getMetadata()
-								.getMessageId()) {
-								// Update Message in LocalDB
-								getChats().get(j).getModel().get(k).getMetadata().setState(returnSync.getMessages().get(j).getMetadata().getState());
+										.getMessageId())
+										chat.getModel().get(j).getMetadata().setState(returnSync.getMessages().get(i).getMetadata().getState());
 						}
-						}
-					}
-				}
-
-			}
-
-			if (returnSync.getMessages().get(i).getMetadata().getMessageId() != 0
-					&& returnSync.getMessages().get(i).getMetadata().getState() == MessageState.READ) {
+						break;
+					case READ:
 						// Update local Messages to state READ
 						logger.info("Message with ID: " + returnSync.getMessages().get(i).getMetadata().getMessageId()
 								+ "was initialized to be set to READ in localDB.");
-				for (int j = 0; j < getChats().size(); j++) {
-					if (getChats().get(j)
-						.getRecipient()
-						.getID() == returnSync.getMessages().get(i).getMetadata().getRecipient()) {
-						logger.info("Chat with: " + getChats().get(j).getRecipient().getID() + "was selected.");
-						for (int k = 0; k < getChats().get(j).getModel().getSize(); k++) {
-							if (getChats().get(j).getModel().get(k).getMetadata().getMessageId() == returnSync.getMessages()
+						for (Chat chat : getChats())
+							if (chat.getRecipient().getID() == returnSync.getMessages().get(i).getMetadata().getRecipient()) {
+								logger.info("Chat with: " + chat.getRecipient().getID() + "was selected.");
+								for (int k = 0; k < chat.getModel().getSize(); k++)
+									if (chat.getModel().get(k).getMetadata().getMessageId() == returnSync.getMessages()
 										.get(i)
 										.getMetadata()
 										.getMessageId()) {
-								logger.info("Message with ID: "
-										+ getChats().get(j).getModel().get(k).getMetadata().getMessageId()
-										+ "was selected.");
-								getChats().get(j)
-									.getModel()
-									.get(k)
-									.getMetadata()
-									.setState(returnSync.getMessages().get(i).getMetadata().getState());
-								logger.info("Message State is now: "
-										+ getChats().get(j).getModel().get(k).getMetadata().getState().toString());
-							}
+										logger.info("Message with ID: " + chat.getModel().get(k).getMetadata().getMessageId() + "was selected.");
+										chat.getModel().get(k).getMetadata().setState(returnSync.getMessages().get(i).getMetadata().getState());
+										logger.info("Message State is now: " + chat.getModel().get(k).getMetadata().getState());
 									}
 							}
+						break;
 				}
 			}
 		}
 
 		// Updating UserStatus of all users in LocalDB
-		for (int j = 0; j < returnSync.getUsers().size(); j++) {
-			for (int k = 0; k < getChats().size(); k++) {
-				if (getChats().get(k).getRecipient().getID() == returnSync.getUsers().get(j).getID()) {
-
-					getChats().get(k).getRecipient().setStatus(returnSync.getUsers().get(j).getStatus());
-					logger.info(getChats().get(k).getRecipient().getStatus().toString());
-				}
-			}
+		for (User user : returnSync.getUsers())
+			for (Chat chat : getChats())
+				if (user.getID() == chat.getRecipient().getID()) {
+					chat.getRecipient().setStatus(user.getStatus());
+					logger.info(chat.getRecipient().getStatus().toString());
 				}
 
 		sync.getMessages().clear();
@@ -241,11 +237,11 @@ public class LocalDB {
 	 * @since Envoy v0.1-alpha
 	 */
 	public void addUnreadMessagesToLocalDB() {
-		Sync unreadMessages = unreadMessagesSync;
-		for (int i = 0; i < unreadMessages.getMessages().size(); i++)
-			for (int j = 0; j < getChats().size(); j++)
-				if (getChats().get(j).getRecipient().getID() == unreadMessages.getMessages().get(i).getMetadata().getSender()) {
-					getChats().get(j).appendMessage(unreadMessages.getMessages().get(i));
+		for (Message message : unreadMessagesSync.getMessages())
+			for (Chat chat : getChats())
+				if (message.getMetadata().getSender() == chat.getRecipient().getID()) {
+					chat.appendMessage(message);
+					break;
 				}
 	}
 
@@ -277,7 +273,6 @@ public class LocalDB {
 		for (Chat chat : getChats())
 			for (int i = 0; i < chat.getModel().size(); i++)
 				if (chat.getModel().get(i).getMetadata().getState() == MessageState.WAITING) {
-					// addMessageToSync(localDB.getChats().get(i).getModel().get(j));
 					logger.info("Got Waiting Message");
 					sync.getMessages().add(chat.getModel().get(i));
 				}
@@ -291,8 +286,8 @@ public class LocalDB {
 	public void clearUnreadMessagesSync() { unreadMessagesSync.getMessages().clear(); }
 
 	/**
-	 * @return all saves {@link Chat} objects that list the client user as the
-	 *         client
+	 * @return all saved {@link Chat} objects that list the client user as the
+	 *         sender
 	 * @since Envoy v0.1-alpha
 	 **/
 	public List<Chat> getChats() { return chats; }

src/main/java/envoy/client/event/Event.java

@@ -0,0 +1,17 @@
+package envoy.client.event;
+
+/**
+ * Project: <strong>envoy-clientChess</strong><br>
+ * File: <strong>Event.javaEvent.java</strong><br>
+ * Created: <strong>04.12.2019</strong><br>
+ * 
+ * @author Kai S. K. Engelbart
+ * @since Envoy v0.2-alpha
+ */
+public interface Event<T> {
+
+	/**
+	 * @return the data associated with this event
+	 */
+	T get();
+}

src/main/java/envoy/client/event/EventBus.java

@@ -0,0 +1,71 @@
+package envoy.client.event;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * This class handles events by allowing {@link EventHandler} object to register
+ * themselves and then be notified about certain events dispatched by the event
+ * bus.<br>
+ * <br>
+ * The event bus is a singleton and can be used across the entire application to
+ * guarantee the propagation of events.
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>EventBus.java</strong><br>
+ * Created: <strong>04.12.2019</strong><br>
+ * 
+ * @author Kai S. K. Engelbart
+ * @since Envoy v0.2-alpha
+ */
+public class EventBus {
+
+	/**
+	 * Contains all {@link EventHandler} instances registered at this
+	 * {@link EventBus}.
+	 */
+	private List<EventHandler> handlers = new ArrayList<>();
+
+	/**
+	 * The singleton instance of this {@link EventBus} that is used across the
+	 * entire application.
+	 */
+	private static EventBus eventBus = new EventBus();
+
+	/**
+	 * This constructor is not accessible from outside this class because a
+	 * singleton instance of it is provided by the {@link EventBus#getInstance()}
+	 * method.
+	 */
+	private EventBus() {}
+
+	/**
+	 * @return the singleton instance of the {@link EventBus}
+	 * @since Envoy v0.2-alpha
+	 */
+	public static EventBus getInstance() { return eventBus; }
+
+	/**
+	 * Registers a list of {@link EventHandler} objects to be notified when a
+	 * {@link Event} is dispatched that they are subscribed to.
+	 * 
+	 * @param handlers the {@link EventHandler} objects to register
+	 * @since Envoy v0.2-alpha
+	 */
+	public void register(EventHandler... handlers) { this.handlers.addAll(Arrays.asList(handlers)); }
+
+	/**
+	 * Dispatches a {@link Event} to every {@link EventHandler} subscribed to it.
+	 * 
+	 * @param event the {@link Event} to dispatch
+	 * @since Envoy v0.2-alpha
+	 */
+	public void dispatch(Event<?> event) { handlers.stream().filter(h -> h.supports().contains(event.getClass())).forEach(h -> h.handle(event)); }
+
+	/**
+	 * @return a list of all {@link EventHandler} instances currently registered at
+	 *         this {@link EventBus}
+	 * @since Envoy v0.2-alpha
+	 */
+	public List<EventHandler> getHandlers() { return handlers; }
+}

src/main/java/envoy/client/event/EventHandler.java

@@ -0,0 +1,25 @@
+package envoy.client.event;
+
+import java.util.Set;
+
+/**
+ * Project: <strong>envoy-clientChess</strong><br>
+ * File: <strong>EventHandler.javaEvent.java</strong><br>
+ * Created: <strong>04.12.2019</strong><br>
+ * 
+ * @author Kai S. K. Engelbart
+ */
+public interface EventHandler {
+
+	/**
+	 * Consumes an event dispatched by the event bus.
+	 * 
+	 * @param event The event dispatched by the event bus, only of supported type
+	 */
+	void handle(Event<?> event);
+
+	/**
+	 * @return A set of classes this class is supposed to handle in events
+	 */
+	Set<Class<? extends Event<?>>> supports();
+}

src/main/java/envoy/client/event/MessageCreationEvent.java

@@ -0,0 +1,18 @@
+package envoy.client.event;
+
+import envoy.schema.Message;
+
+/**
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>MessageCreationEvent.java</strong><br>
+ * Created: <strong>4 Dec 2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ */
+public class MessageCreationEvent extends MessageEvent {
+
+	/**
+	 * @param message the {@link Message} that has been created
+	 */
+	public MessageCreationEvent(Message message) { super(message); }
+}

src/main/java/envoy/client/event/MessageEvent.java

@@ -0,0 +1,20 @@
+package envoy.client.event;
+
+import envoy.schema.Message;
+
+/**
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>MessageCreationEvent.java</strong><br>
+ * Created: <strong>4 Dec 2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ */
+public class MessageEvent implements Event<Message> {
+
+	protected final Message message;
+
+	public MessageEvent(Message message) { this.message = message; }
+
+	@Override
+	public Message get() { return message; }
+}

src/main/java/envoy/client/event/MessageModificationEvent.java

@@ -0,0 +1,18 @@
+package envoy.client.event;
+
+import envoy.schema.Message;
+
+/**
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>MessageModificationEvent.java</strong><br>
+ * Created: <strong>4 Dec 2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ */
+public class MessageModificationEvent extends MessageEvent {
+
+	/**
+	 * @param message the {@link Message} that has been modified
+	 */
+	public MessageModificationEvent(Message message) { super(message); }
+}

src/main/java/envoy/client/ui/ChatWindow.java

@@ -125,13 +125,11 @@ public class ChatWindow extends JFrame {
 
 			@Override
 			public void keyReleased(KeyEvent e) {
-
-				if (e.getKeyCode() == KeyEvent.VK_ENTER && ((SettingsScreen.enterToSend && e.getModifiersEx() == 0)
-						|| (e.getModifiersEx() == KeyEvent.CTRL_DOWN_MASK))) {
+				if (e.getKeyCode() == KeyEvent.VK_ENTER
+						&& ((SettingsScreen.enterToSend && e.getModifiersEx() == 0) || (e.getModifiersEx() == KeyEvent.CTRL_DOWN_MASK))) {
 
 					postMessage(messageList);
 				}
-
 			}
 		});
 		// Checks for changed Message
@@ -219,11 +217,7 @@ public class ChatWindow extends JFrame {
 				final User			user				= selectedUserList.getSelectedValue();
 				client.setRecipient(user);
 
-				currentChat = localDB.getChats()
-					.stream()
-					.filter(chat -> chat.getRecipient().getID() == user.getID())
-					.findFirst()
-					.get();
+				currentChat = localDB.getChats().stream().filter(chat -> chat.getRecipient().getID() == user.getID()).findFirst().get();
 
 				// Set all unread messages in the chat to read
 				readCurrentChat();
@@ -262,10 +256,8 @@ public class ChatWindow extends JFrame {
 
 	private void postMessage(JList<Message> messageList) {
 		if (!client.hasRecipient()) {
-			JOptionPane.showMessageDialog(this,
-					"Please select a recipient!",
-					"Cannot send message",
-					JOptionPane.INFORMATION_MESSAGE);
+			JOptionPane.showMessageDialog(this, "Please select a recipient!", "Cannot send message", JOptionPane.INFORMATION_MESSAGE);
+			return;
 		}
 
 		if (!messageEnterTextArea.getText().isEmpty()) try {
@@ -321,8 +313,7 @@ public class ChatWindow extends JFrame {
 			new Thread(() -> {
 
 				// Synchronize
-				localDB.applySync(
-						client.sendSync(client.getSender().getID(), localDB.fillSync(client.getSender().getID())));
+				localDB.applySync(client.sendSync(client.getSender().getID(), localDB.fillSync(client.getSender().getID())));
 
 				// Process unread messages
 				localDB.addUnreadMessagesToLocalDB();
@@ -332,8 +323,7 @@ public class ChatWindow extends JFrame {
 				readCurrentChat();
 
 				// Update UI
-				SwingUtilities
-					.invokeLater(() -> { updateUserStates(); contentPane.revalidate(); contentPane.repaint(); });
+				SwingUtilities.invokeLater(() -> { updateUserStates(); contentPane.revalidate(); contentPane.repaint(); });
 			}).start();
 		}).start();
 	}

src/main/java/envoy/client/ui/Startup.java

@@ -72,8 +72,9 @@ public class Startup {
 
 		EventQueue.invokeLater(() -> {
 			try {
-				ChatWindow frame = new ChatWindow(client, localDB);
-				frame.setVisible(true);
+				ChatWindow chatWindow = new ChatWindow(client, localDB);
+				new StatusTrayIcon(chatWindow).show();
+				chatWindow.setVisible(true);
 			} catch (Exception e) {
 				e.printStackTrace();
 			}

src/main/java/envoy/client/ui/StatusTrayIcon.java

@@ -0,0 +1,133 @@
+package envoy.client.ui;
+
+import java.awt.AWTException;
+import java.awt.Image;
+import java.awt.MenuItem;
+import java.awt.PopupMenu;
+import java.awt.SystemTray;
+import java.awt.Toolkit;
+import java.awt.TrayIcon;
+import java.awt.TrayIcon.MessageType;
+import java.awt.Window;
+import java.awt.event.WindowAdapter;
+import java.awt.event.WindowEvent;
+import java.util.HashSet;
+import java.util.Set;
+
+import envoy.client.event.Event;
+import envoy.client.event.EventBus;
+import envoy.client.event.EventHandler;
+import envoy.client.event.MessageCreationEvent;
+import envoy.exception.EnvoyException;
+import envoy.schema.Message;
+
+/**
+ * Project: <strong>envoy-client</strong><br>
+ * File: <strong>StatusTrayIcon.java</strong><br>
+ * Created: <strong>3 Dec 2019</strong><br>
+ *
+ * @author Kai S. K. Engelbart
+ * @since Envoy v0.2-alpha
+ */
+public class StatusTrayIcon implements EventHandler {
+
+	/**
+	 * The {@link TrayIcon} provided by the System Tray API for controlling the
+	 * system tray. This includes displaying the icon, but also creating
+	 * notifications when new messages are received.
+	 */
+	private TrayIcon trayIcon;
+
+	/**
+	 * A received {@link Message} is only displayed as a system tray notification if
+	 * this variable is set to {@code true}.
+	 */
+	private boolean displayMessages = false;
+
+	/**
+	 * Creates a {@link StatusTrayIcon} with the Envoy logo, a tool tip and a pop-up
+	 * menu.
+	 *
+	 * @param focusTarget the {@link Window} which focus determines if message
+	 *                    notifications are displayed
+	 * @throws EnvoyException if the currently used OS does not support the System
+	 *                        Tray API
+	 * @since Envoy v0.2-alpha
+	 */
+	public StatusTrayIcon(Window focusTarget) throws EnvoyException {
+		if (!SystemTray.isSupported()) throw new EnvoyException("The Envoy tray icon is not supported.");
+
+		ClassLoader	loader	= Thread.currentThread().getContextClassLoader();
+		Image		img		= Toolkit.getDefaultToolkit().createImage(loader.getResource("envoy_logo.png"));
+		trayIcon = new TrayIcon(img, "Envoy Client");
+		trayIcon.setImageAutoSize(true);
+		trayIcon.setToolTip("You are notified if you have unread messages.");
+
+		PopupMenu popup = new PopupMenu();
+
+		MenuItem exitMenuItem = new MenuItem("Exit");
+		exitMenuItem.addActionListener((evt) -> System.exit(0));
+		popup.add(exitMenuItem);
+
+		trayIcon.setPopupMenu(popup);
+
+		// Only display messages if the chat window is not focused
+		focusTarget.addWindowFocusListener(new WindowAdapter() {
+
+			@Override
+			public void windowGainedFocus(WindowEvent e) {
+				displayMessages = false;
+			}
+
+			@Override
+			public void windowLostFocus(WindowEvent e) {
+				displayMessages = true;
+			}
+		});
+
+		// Start processing message events
+		EventBus.getInstance().register(this);
+	}
+
+	/**
+	 * Makes this {@link StatusTrayIcon} appear in the system tray.
+	 *
+	 * @throws EnvoyException if the status icon could not be attaches to the system
+	 *                        tray for system-internal reasons
+	 * @since Envoy v0.2-alpha
+	 */
+	public void show() throws EnvoyException {
+		try {
+			SystemTray.getSystemTray().add(trayIcon);
+		} catch (AWTException e) {
+			throw new EnvoyException("Could not attach Envoy tray icon to system tray.", e);
+		}
+	}
+
+	/**
+	 * Notifies the user of a message by displaying a pop-up every time a new
+	 * message is received.
+	 *
+	 * @since Envoy v0.2-alpha
+	 */
+	@Override
+	public void handle(Event<?> event) {
+		System.out.println("Message received. Displaying message: " + displayMessages);
+		if (displayMessages)
+			trayIcon.displayMessage("New message received", ((MessageCreationEvent) event).get().getContent().get(0).getText(), MessageType.INFO);
+	}
+
+	/**
+	 * The {@link StatusTrayIcon} only reacts to {@link MessageCreationEvent}
+	 * instances which signify newly received messages.
+	 *
+	 * @return A set with the single element {@code MessageCreationEvent.class}
+	 * @since Envoy v0.2-alpha
+	 */
+	@Override
+	public Set<Class<? extends Event<?>>> supports() {
+		Set<Class<? extends Event<?>>> supportedEvents = new HashSet<>();
+		supportedEvents.add(MessageCreationEvent.class);
+		return supportedEvents;
+	}
+}