Update Ui class with user greeting functionality and add BCD Extension.

greetUser() method functionality added to Ui class.
Adding greeting here makes code more systematic and
concise.

Let's add a static greetUser() method.

Doing so will make debugging easier, cause lesser confusion
and improve readability.

src/main/java/ui/Ui.java

@@ -12,9 +12,14 @@
  */
 public class Ui {
 
-    TaskList tasks;
-    NoteList notes;
+    private TaskList tasks;
+    private NoteList notes;
 
+    /**
+     * Ui class constructor.
+     * @param tasks contains list of all tasks added by user.
+     * @param notes contains list of all notes added by user.
+     */
     public Ui(TaskList tasks, NoteList notes) {
         this.tasks = tasks;
         this.notes = notes;
@@ -36,32 +41,36 @@ public String startConversation(Parser parser, Storage storage, String userInput
      * @return String message to greet user when the bot is started.
      */
     public static String greetUser() {
-        return "Hello! I'm Duke :) \nWhat can I do for you? :D\n\n" +
-                "Enter 'help' to view Duke commands...";
+        return "Hello! I'm Duke :) \nWhat can I do for you? :D\n\n"
+                + "Enter 'help' to view Duke commands...";
     }
 
+    /**
+     * Provides help to user by listing duke commands.
+     * @return String containing all duke commands.
+     */
     public static String userHelp() {
-        return "Duke commands:\n" +
-                "To access notes, start command with 'note'. To access tasks start command with 'task'.\n\n" +
-                "Task feature example commands:\n" +
-                "1. Creating todos: task new todo <todo description>\n" +
-                "2. Creating deadlines: task new deadline <deadline description> by <date in YYYY-MM-DD format>\n" +
-                "3. Creating events: task new event <event description> on <date in YYYY-MM-DD format>\n" +
-                "4. Viewing task list: task list\n" +
-                "5. Marking tasks as complete: task mark <index number of task to mark>\n" +
-                "6. Marking tasks as incomplete: task unmark: <index number of task to unmark>\n" +
-                "7. Find a tasks by keyword: task find <keyword>\n" +
-                "8. Deleting a task by index number: task delete <index number>\n" +
-                "9. Deleting all tasks: task delete all\n\n" +
-                "Note feature example commands:\n" +
-                "1. Creating new note: note new <note description>\n" +
-                "2. Viewing all notes: note list\n" +
-                "3. Finding a note by keyword: note find <keyword>\n" +
-                "4. Deleting a note by index number: note delete <index number>\n" +
-                "5. Deleting all notes: note delete all\n\n" +
-                "Other commands:\n" +
-                "1. Viewing this help section: help\n" +
-                "2. Closing duke conversation: bye";
+        return "Duke commands:\n"
+                + "To access notes, start command with 'note'. To access tasks start command with 'task'.\n\n"
+                + "Task feature example commands:\n"
+                + "1. Creating todos: task new todo <todo description>\n"
+                + "2. Creating deadlines: task new deadline <deadline description> by <date in YYYY-MM-DD format>\n"
+                + "3. Creating events: task new event <event description> on <date in YYYY-MM-DD format>\n"
+                + "4. Viewing task list: task list\n"
+                + "5. Marking tasks as complete: task mark <index number of task to mark>\n"
+                + "6. Marking tasks as incomplete: task unmark: <index number of task to unmark>\n"
+                + "7. Find a tasks by keyword: task find <keyword>\n"
+                + "8. Deleting a task by index number: task delete <index number>\n"
+                + "9. Deleting all tasks: task delete all\n\n"
+                + "Note feature example commands:\n"
+                + "1. Creating new note: note new <note description>\n"
+                + "2. Viewing all notes: note list\n"
+                + "3. Finding a note by keyword: note find <keyword>\n"
+                + "4. Deleting a note by index number: note delete <index number>\n"
+                + "5. Deleting all notes: note delete all\n\n"
+                + "Other commands:\n"
+                + "1. Viewing this help section: help\n"
+                + "2. Closing duke conversation: bye";
     }
 
     /**