Aren’t we breaking the Single Responsibility Principle with this super-mega-god-interface?

Not exactly, because the interface isn’t doing anything. It has no responsibilities.

You are breaking Liskov Substitution Principle and Interface Segregation Principle though.

LSP because you’re writing methods that do nothing, just to make it fit the interface (ie. What is true of IDrawable — that you can move it — is not true of Wall).

ISP because you’ll have situations like a trade method, which should have access to Buy and Sell methods, but will also have access to Draw.

Are both approaches valid against the Liskov’s Substitution Principle? I think it’s broken in the first case because the post-conditions are broken due to some methods don’t do anything. Anyway, in the second approach I’m not sure as well because of the interfaces inheritance tree

It’s just as broken in the second case as the first.

Edit: On second thoughts, it isn’t. But it is still broken. It doesn’t seem automatically true that everything that could have a Move method will also have a Draw method. It might be true in your case but, if you later find yourself wanting to pass an object with a Move method into a method that receives an IMovable, you’re going to have to implement Draw on that object, even if you don’t need it.

And it gains you nothing over a third solution …

Maybe, another alternative could be the definition of basic Drawable objects (with ‘click’ and ‘draw’ methods). Thus, we may take advantage of some kind of mechanism like the Decorator pattern in order to add behaviors dynamically. But how?

The best alternative is to segregate your interfaces fully. You can still have objects with lots of methods (but Dependency-Inversion Principle and Single Responsibility Principle dictate you must push the logic down into services), but they only have to implement the interfaces they need. Wall shouldn’t implement IMovable, if it can’t be moved.

public class Wall : IDrawable
public class Furniture : IDrawable, IMovable, ISellable
public class Window : IDrawable, IOpenable

Nothing here dictates that any of those interfaces must derive from another.

public interface IDrawable
{
    void Draw(Canvas canvas);
}

public interface IMovable
{
    void MoveTo(int x, int y);
}

etc

You should then manage your list of IDrawables, which you can only be sure implement Draw(), and cast to see if other methods are available.

For example,

ITradable tradable = drawable as ITradable;
if (tradable != null)
{
    tradable.Trade(buyer, seller);
}

You’re going to have to do the above in your option 2 anyway, so why tie everything to IDrawable?

If we use a language without interfaces such as JavaScript or Python, how can we deal with this problem?

OO languages tend to support at least one of three things:

• Interfaces (use … interfaces)
• Multiple inheritance (use abstract classes)
• Dynamic types (use duck-typing; this is where Python comes in)

Prototypical languages, such as Javascript, support … well, prototypes, which allow for a slightly modified version of duck-typing (in prototypes, you generally ask if a method exists before calling it).

Other paradigms generally handle these issues in their own way.

1. Solution 1 violates not only Liskov’s Substitution Principle but it also violates the Interface Segregation Principle and the Single Responsability Principle. Solution 2 is the correct approach (see correction at the bottom).

2. Same as above

3. You can do that and solution 2 at the same time, they are not exclusive.

4. You should use non-OO solutions and patterns.

CORRECTION: Solution 2 is also wrong. Sellable and Movable must not inherit from Drawable. They should be separate and unrelated interface.