Add a new command (show total number of persons in the address book)

src/seedu/addressbook/commands/Command.java

@@ -36,6 +36,11 @@ public static String getMessageForPersonListShownSummary(List<? extends ReadOnly
         return String.format(Messages.MESSAGE_PERSONS_LISTED_OVERVIEW, personsDisplayed.size());
     }
 
+    /* get message of total number */
+    public static String getMessageForTotalNumber(List<? extends ReadOnlyPerson> personsDisplayed) {
+        return String.format(Messages.MESSAGE_TOTAL, personsDisplayed.size());
+    }
+
     /**
      * Executes the command and returns the result.
      */

src/seedu/addressbook/commands/TotalNumberCommand.java

@@ -0,0 +1,25 @@
+package seedu.addressbook.commands;
+
+import seedu.addressbook.data.person.ReadOnlyPerson;
+
+import java.util.List;
+
+
+/**
+ * Lists all persons in the address book to the user.
+ */
+public class TotalNumberCommand extends Command {
+
+    public static final String COMMAND_WORD = "Total";
+
+    public static final String MESSAGE_USAGE = COMMAND_WORD
+            + ": Displays total number of all the people in this address book.\n"
+            + "Example: " + COMMAND_WORD;
+
+
+    @Override
+    public CommandResult execute() {
+        List<ReadOnlyPerson> allPersons = addressBook.getAllPersons().immutableListView();
+        return new CommandResult(getMessageForTotalNumber(allPersons), null);
+    }
+}

src/seedu/addressbook/common/Messages.java

@@ -13,6 +13,7 @@ public class Messages {
     public static final String MESSAGE_PERSONS_LISTED_OVERVIEW = "%1$d persons listed!";
     public static final String MESSAGE_PROGRAM_LAUNCH_ARGS_USAGE = "Launch command format: " +
             "java seedu.addressbook.Main [STORAGE_FILE_PATH]";
+    public static final String MESSAGE_TOTAL = "%1$d persons in the address book!";
     public static final String MESSAGE_WELCOME = "Welcome to your Address Book!";
     public static final String MESSAGE_USING_STORAGE_FILE = "Using storage file : %1$s";
 }

src/seedu/addressbook/parser/Parser.java

@@ -20,6 +20,7 @@
 import seedu.addressbook.commands.HelpCommand;
 import seedu.addressbook.commands.IncorrectCommand;
 import seedu.addressbook.commands.ListCommand;
+import seedu.addressbook.commands.TotalNumberCommand;
 import seedu.addressbook.commands.ViewAllCommand;
 import seedu.addressbook.commands.ViewCommand;
 import seedu.addressbook.data.exception.IllegalValueException;
@@ -76,6 +77,9 @@ public Command parseCommand(String userInput) {
         case AddCommand.COMMAND_WORD:
             return prepareAdd(arguments);
 
+            case TotalNumberCommand.COMMAND_WORD:
+                return  new TotalNumberCommand();
+
         case DeleteCommand.COMMAND_WORD:
             return prepareDelete(arguments);