How and when to use an abstract class

Question

You’d use an abstract class or interface here in order to make a common base class/interface that provides the void draw() method, e.g.

abstract class Shape() {
  void draw();
}

class Circle extends Shape {
   void draw() { ... }
}

...

Shape s = new Circle();
s.draw();

I’d generally use an interface. However you might use an abstract class if:

You need/want to provide common functionality or class members (e.g. the int i member in your case).
Your abstract methods have anything other than public access (which is the only access type allowed for interfaces), e.g. in my example, void draw() would have package visibility.