class BaseGeometry {

hello,

oh, the forum just killed my old long answer

in short: read tutorials 1-3 and 5 here (mainly 5):
http://www.processing.org/tutorials/

Greetings, Chrisir    

As said in the assignment, Drawable must be an interface: replace the class keyword with the interface one, and remove the body of the method:

1. interface Drawable
2. {
3.   void draw();
4. }

This means, for classes implementing this interface, that they must have a draw() method of same signature.

1. class SomeShape extends Polygon implements Drawable
2. {
3.   // Here, fields, constructor(s), methods
4.   // ...
5.   void draw()
6.   {
7.     // Use x, y, inside, stroke to draw something
8.   }
9. }

One interest of interfaces is to ensure that classes of various kind have the wanted methods: you are sure you can call a draw() method on them.
Another interest is that you can put these classes in a collection:

1. Drawable someShape = new SomeShape();
2. Drawable ellipse = new Ellipse();
3. ArraList<Drawable> drawables = new ArrayList<Drawable>();
4. drawables.add(someShape);
5. drawables.add(ellipse);
6. for (Drawable drawable : drawables)
7. {
8.   drawable.draw();
9. }

It allows to apply an uniform treatment to a collection of otherwise various classes, using their common point.