Java Development Tools

In the Toy Store Demo application, there is a loginform.java action class, which returns an error:
...
errors.add("username", new ActionError("signin.error.usernameblank"));

This will be displayed in the signin.jsp by:
...
<html:errors property="username"/>


Now, if, instead, I want to display the trapped error message using a message box (like the MS windows standard message popup),
how can I do it?