package savvytodo.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$s";
    public static final String MESSAGE_INVALID_TASK_DISPLAYED_INDEX = "The task index provided is invalid";
    public static final String MESSAGE_TASK_LISTED_OVERVIEW = "%1$d tasks listed!";
    public static final String MESSAGE_INVALID_DATETIME = "Invalid date time entered!";
    public static final String MESSAGE_CONFLICTING_TASKS_WARNING = "Warning! DateTime conflict with: ";

}

package savvytodo.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$s";
    public static final String MESSAGE_INVALID_TASK_DISPLAYED_INDEX = "The task index provided is invalid";
    public static final String MESSAGE_TASK_LISTED_OVERVIEW = "%1$d tasks listed!";
    public static final String MESSAGE_CATEGORY_NOT_EXISTS = "The category does not exist";
    public static final String MESSAGE_INVALID_DATETIME = "Invalid date time entered!";
    public static final String MESSAGE_CONFLICTING_TASKS_WARNING = "Warning! DateTime conflict with: ";

}

Update Messages.java