Declaring top level classes instead of subclasses

Question

I seem to have misunderstood something very basic.
It's always better, where possible, to declare the top-level class instead of the sub-class, right?
Ok, then why do I have the following problem?
I declare, from the JavaMail API the following:
Message theMessage = new MimeMessage(session);instead of declaring directly the subclass like this:
MimeMessage theMessage = new MimeMessage(session);but when I use a method which is only found in the MimeMessage class I get a compile error.
[javac] 189.           theMessage.setSubject(subject, "iso8859_1");
[javac] <-------------------------------->
[javac] *** Error: No match was found for method "setSubject(java.lang.String, java.lang.String)".
Have I really misunderstood something so basic?? Surely the compiler knows that theMessage is an instance of MimeMessage??
Any clear explanation to explain why I get this compile error would be gratefully received. I have been faithfully declaring top-level classes instead of subclasses in my code, but if the compiler doesn't let you do so, then what's the point?

Answer 1

Ok, my confusion was started by an article on the
Collection classes which stated that:
Map theMap = new HashMap(); was better
programming practice.
Yep.
But the Map class is an Interface, right? - Whereas
Message is a normal class with subclass MimeMessage.
I think that's where my confusion started.Well, this isn't really a class vs. interface difference. It goes to which class (or interface) is the "highest" (closest to Object) and still has all the public interface you need.
There are cases where you'll declare a variable to be of a base class type like Message.
You just need to get into the habit of separating the left side of the = from the right side. On the left, you put the bare minimum that completely meets your needs. You say, "I need a List" or "I need a Map" or "I need a SortedMap" (a subinterface of Map) or "I need a Message" or "I need a MimeMesage".
Then, on the RHS =, you decide which particular class best implements the public interface that you said you need (by declaring it on the LHS). "For my Map, I'll use a HashMap." "For my Message, I'll use a MimeMessage." "For my MimeMessage, I'll use a MimeMessage".