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

you could do

Ibox myBox = new Rectangle();

that way you are using this object as Ibox and you don't care that its really Rectangle .