Is there more to an interface than having the correct methods

Question

So lets say I have this interface:

public interface IBox
{
   public void setSize(int size);
   public int getSize();
   public int getArea();
  //...and so          


        

Answer 1

If you have CardboardBox and HtmlBox (both of which implement IBox), you can pass both of them to any method that accepts a IBox. Even though they are both very different and not completely interchangable, methods that don't care about "open" or "resize" can still use your classes (perhaps because they care about how many pixels are needed to display something on a screen).