Add examples of lower-level log messages

Let's add a few examples of lower-level log messages, to illustrate to
developers how they can be used in this codebase.

src/main/java/seedu/address/MainApp.java

@@ -74,6 +74,8 @@ public void init() throws Exception {
      * or an empty address book will be used instead if errors occur when reading {@code storage}'s address book.
      */
     private Model initModelManager(Storage storage, ReadOnlyUserPrefs userPrefs) {
+        logger.info("Using data file : " + storage.getAddressBookFilePath());
+
         Optional<ReadOnlyAddressBook> addressBookOptional;
         ReadOnlyAddressBook initialData;
         try {

src/main/java/seedu/address/logic/parser/AddressBookParser.java

@@ -3,9 +3,11 @@
 import static seedu.address.logic.Messages.MESSAGE_INVALID_COMMAND_FORMAT;
 import static seedu.address.logic.Messages.MESSAGE_UNKNOWN_COMMAND;
 
+import java.util.logging.Logger;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
+import seedu.address.commons.core.LogsCenter;
 import seedu.address.logic.commands.AddCommand;
 import seedu.address.logic.commands.ClearCommand;
 import seedu.address.logic.commands.Command;
@@ -26,6 +28,7 @@ public class AddressBookParser {
      * Used for initial separation of command word and args.
      */
     private static final Pattern BASIC_COMMAND_FORMAT = Pattern.compile("(?<commandWord>\\S+)(?<arguments>.*)");
+    private static final Logger logger = LogsCenter.getLogger(AddressBookParser.class);
 
     /**
      * Parses user input into command for execution.
@@ -42,6 +45,12 @@ public Command parseCommand(String userInput) throws ParseException {
 
         final String commandWord = matcher.group("commandWord");
         final String arguments = matcher.group("arguments");
+
+        // Note to developers: Change the log level in config.json to enable lower level (i.e., FINE and lower)
+        // log messages such as the one below.
+        // Currently, lower level log messages are used sparingly, to minimize noise in the code.
+        logger.fine("Command word: " + commandWord + " ; Arguments: " + arguments);
+
         switch (commandWord) {
 
         case AddCommand.COMMAND_WORD:
@@ -69,6 +78,7 @@ public Command parseCommand(String userInput) throws ParseException {
             return new HelpCommand();
 
         default:
+            logger.finer("This user input caused a ParseException: " + userInput);
             throw new ParseException(MESSAGE_UNKNOWN_COMMAND);
         }
     }