Subclass with interface can't be used in place of superclass?

Question

So I've searched through the forums and haven't found much of a real answer to this yet--at least, the keywords I tried resulted in nothing. This should also be a quickie--it seems pretty simple, but I'm stumped after spending so long working.
So the problem is as follows. I need to ensure that certain Swing components in my application always implement a certain set of methods, so naturally this lends itself to an interface. I don't think an abstract class would be necessary in this case since these objects are going to be pretty radically different from each other in terms of function.
I create my interface, we'll call it "Presentation". I create an object "PaintingPanel" which implements the interface, and since I'll need to add it to my Swing application, I have it extend JLabel, or some other JComponent.
I thought this would work, however, JBuilder complains about the following lines of code. The instantiation is fine, but adding it gives me trouble.
Presentation myPres = new PaintingPanel();
myJPanel.add(myPres);What am I doing wrong exactly? Should I scrap the interface then and go with an abstract class? I can't just cast this, can I?
Thanks in advance.

Answer 1

I don't have a "this is the right way to do it" answer for you, but look at it this way.
First, you're saying "Presentation myPres..." so all you're promising about myPres is that it "is-a" Presentation, and that it "is-a" everything the Presentation extends/implements.
Then you want to call JPanel.add(myPres), but JPanel.add requires a Component (I think). As far as the compiler knows, myPres is a Presentation, which, since it doesn't extend Component, "isn't-a" Component.
There are a couple things you could do.
* Declare it as MyClassThatImpelmentsPresentationAndExtendsComponent or whatever it is. If you're expecting to treat this thing as both a Presentation and a Component, this seems to make the most sense.
* Cast it, as stevejluke suggested.
* Call add(myPres.getComponent()), where the getComponent is a method you add in case you[i] don't want all your Presentation implementations to have to extend Component, but[i] do  want to be able to use the Presentation (or something associated with it) in a Component context. For those Presentation implementations that do extend Component, you can just have the method return this.
There are probably other options, depending on your actual requirements. No matter what though, the key is to be driven by the fact that add requires a Component, so you need some way to guarantee the compiler that that's what you're passing.