basic oo question

Assuming that I've a simply emailAddresss class that looks like below

public class EmailAddress {

private String value = "";

public EmailAddress( String newValue )
setValue( newValue );
}
. . .
}


Assuming that I now need to be able valid the emailAddress to make sure the value contains a valid email address. In OO, what is the normal way to handle this:
-> do I change the constructor to now throws an exception when newValue is not in valid email address format.
or
-> do I add a public method in this class that can be executed to determine if value contains valid email address after the EmailAddress object is created.
or
-> Do I create a new validation class that validate the EmailAddress

thanks