package seedu.watodo.commons.core;

/**
 * Container for user visible messages.
 */
public class Messages {

    public static final String MESSAGE_UNKNOWN_COMMAND = "Unknown command";
    public static final String MESSAGE_INVALID_COMMAND_FORMAT = "The task index provided is invalid \n%1$s";
    public static final String MESSAGE_INVALID_TASK_DISPLAYED_INDEX = "The task index provided is invalid";
    public static final String MESSAGE_TASKS_LISTED_OVERVIEW = "%1$d tasks listed!";

}

package seedu.watodo.commons.core;

/**
 * Container for user visible messages.
 */
public class Messages {

    public static final String MESSAGE_UNKNOWN_COMMAND = "Unknown command";
    public static final String MESSAGE_INVALID_COMMAND_FORMAT = "Invalid command format! \n%1$s";
    public static final String MESSAGE_INVALID_TASK_DISPLAYED_INDEX = "The task index provided is invalid";
    public static final String MESSAGE_TASKS_LISTED_OVERVIEW = "%1$d tasks listed!";

}

Update Messages.java