Revised message related classes

Added GroupMessage class as subclass from Message and adjusted the
MessageBuilder

src/main/java/envoy/data/GroupMessage.java

@@ -0,0 +1,46 @@
+package envoy.data;
+
+import java.util.Date;
+import java.util.Map;
+
+/**
+ * Project: <strong>envoy-common</strong><br>
+ * File: <strong>GroupMessage.java</strong><br>
+ * Created: <strong>26.03.2020</strong><br>
+ * 
+ * @author Maximilian K&auml;fer
+ * @since Envoy Common v0.1-beta
+ */
+public final class GroupMessage extends Message {
+
+	private final Map<Long, MessageStatus> memberStatuses;
+
+	private static final long serialVersionUID = 0L;
+
+	/**
+	 * Initializes a {@link GroupMessage} with values for all of its properties. The
+	 * use
+	 * of this constructor is only intended for the {@link MessageBuilder} class, as
+	 * this class provides {@code null} checks and default values for all
+	 * properties.
+	 *
+	 * @param id             unique ID
+	 * @param senderID       the ID of the user who sends the message
+	 * @param recipientID    the ID of the user who receives the message
+	 * @param creationDate   the creation date of the message
+	 * @param text           the text content of the message
+	 * @param attachment     the attachment of the message, if present
+	 * @param status         the current {@link MessageStatus} of the message
+	 * @param forwarded      whether this message was forwarded
+	 * @param memberStatuses a map of all members and their status according to this
+	 *                       {@link GroupMessage}
+	 * @since Envoy Common v0.1-beta
+	 */
+	GroupMessage(long id, long senderID, long groupID, Date creationDate, String text, MessageAttachment<?> attachment, MessageStatus status,
+			boolean forwarded, Map<Long, MessageStatus> memberStatuses) {
+		super(id, senderID, groupID, creationDate, text, attachment, status, forwarded);
+		this.memberStatuses = memberStatuses;
+	}
+
+	public Map<Long, MessageStatus> getMemberStatuses() { return memberStatuses; }
+}

src/main/java/envoy/data/MessageBuilder.java

@@ -1,6 +1,7 @@
 package envoy.data;
 
 import java.util.Date;
+import java.util.HashMap;
 
 import envoy.data.Message.MessageStatus;
 
@@ -97,12 +98,22 @@ public class MessageBuilder {
 	 * @since Envoy Common v0.2-alpha
 	 */
 	public Message build() {
-		// Supplement default values
+		supplyDefaults();
+		return new Message(id, senderID, recipientID, creationDate, text, attachment, status, forwarded);
+	}
+
+	public GroupMessage buildGroupMessage(Group group) {
+		if (group == null) throw new NullPointerException();
+		supplyDefaults();
+		var memberStatuses = new HashMap<Long, Message.MessageStatus>();
+		group.getMemberIDs().forEach(id -> memberStatuses.put(id, MessageStatus.WAITING));
+		return new GroupMessage(id, senderID, recipientID, creationDate, text, attachment, status, forwarded, memberStatuses);
+	}
+
+	private void supplyDefaults() {
 		if (creationDate == null) creationDate = new Date();
 		if (text == null) text = "";
 		if (status == null) status = MessageStatus.WAITING;
-
-		return new Message(id, senderID, recipientID, creationDate, text, attachment, status, forwarded);
 	}
 
 	/**