When generating error messages, two Java utility classes, ResourceBundle and MessageFormat, are extremely practical and powerful.  From the ResourceBundle JavaDoc:

Resource bundles contain locale-specific objects. When your program needs a locale-specific resource, a String for example, your program can load it from the resource bundle that is appropriate for the current user’s locale. In this way, you can write program code that is largely independent of the user’s locale isolating most, if not all, of the locale-specific information in resource bundles.

This allows you to write programs that can:

• be easily localized, or translated, into different languages
• handle multiple locales at once
• be easily modified later to support even more locales

And from the MessageFormat JavaDoc:

MessageFormat provides a means to produce concatenated messages in a language-neutral way. Use this to construct messages displayed for end users.

MessageFormat takes a set of objects, formats them, then inserts the formatted strings into the pattern at the appropriate places.

This is an example of an error message resource bundle, ErrorMessagesResources.properties.

userAlreadyExists=A user already exists with the name {0}.
passwordInvalid=Please enter a valid password.

This is an example of how you would use this resource bundle.

protected static final ResourceBundle resourceBundle =
    ResourceBundle.getBundle("com.betweengo.ErrorMessageResources");

public boolean handleLogin(DynamoHttpServletRequest pReq, DynamoHttpServletResponse pRes) {

  ...

  // user already exists
  String errMsg1 = resourceBundle.getString("userAlreadyExists");
  errMsg1 = MessageFormat.format(errMsg1, userName);

  ...

  // password invalid
  String errMsg2 = resourceBundle.getString("passwordInvalid");

  ...  
}