I was recently reminded of a drag and drop project I'd worked on in Actionscript and wondered how I would implement it from scratch in Processing.  I also saw this as an opportunity to practice OOP techniques, in particular 'Inheritance'.  It's still got some bugs and things I'd like to improve but I thought I'd post it here and get some feedback from the OOP gurus

It's not the most exciting thing to look at...

http://www.blindfish.co.uk/code/processing/drag-drop/

I'm particularly interested in any ways I might optimise the code and how I would implement appropriate Interfaces.

Hmm... the shadow is just a drop shadow added to the PNG in photoshop; so nothing special

'moveToTarget' is a method in the CircleDragger class: simple but effective easing animation.  There are also methods to draw each state (default, mouse-over etc.); so if you subclass it you have the option to draw whatever you like by overriding the appropriate method.  If you don't override it simply uses the default.

This little project has certainly opened my eyes to the benefits of OOP, and I can already see some improvements, but I'm not sure what effect these might have on performance.  For instance if I were to add properties to my CircleDragger class (to allow changes to colours by setting appropriate properties) and someone decided to subclass this but not use those properties (e.g. using images instead, as in the crudely implemented 'MyCircleDragger') would the fact that the class has these, unused, properties mean it has a larger memory footprint; or is memory only allocated if the properties are actually populated?

I also see the benefits of Inheritance, but there are also benefits to having parent classes (e.g. the reverting to default display behaviour described above; referencing the parent class to store different sub-objects in a single array etc.).  As such would it make sense to extend one class but also implement an Interface?  Presumably that would ensure required methods were implemented whilst retaining some of the default, useful behaviour...

I'd more or less understood the idea of an interface but was a little confused as to whether that was the best option in my case.  I've actually discovered that a better approach was to define my base classes as abstract - they're not meant to be instantiated anyway, and I'm able to declare both methods that will be inherited and abstract methods that must be implemented in the child classes.  I've now updated the drag and drop demo with these improvements.

I've still got a bug to fix and I still have one OOP-related question:

I want to give people the option to pass a single target or an array of targets to my dragger objects.  I have set up two constructors to handle this in my BaseDragger class, but it seems that I also need to create duplicate constructors in child methods to accept both a single target or array of targets as parameters.  Presumably there's no way of avoiding this

As it happens it's just occurred to me that I can write a private method to handle generic instantiation and call this from the separate constructors to make the process of writing all the constructors simpler; but if there's a workaround I'd love to hear about it.

I've just uploaded my latest (and probably final) version.  I've now added a class to keep track of whether draggers have been places correctly and when all targets are covered.

By the standards of things posted here usually I know this isn't the most exciting sketch, but it's been a really useful way for me to get my head round some OOP concepts and hopefully it will prove useful to someone out there

Edit - Sorry: forgot to thank PhilHo for the tip on Observer and Listener patterns