Ok, I am stupid, I am new to this, I get that....but what the heck?! According to this article ( that I read over and over again ) https://codegym.cc/groups/posts/104-the-difference-between-abstract-classes-and-interfaces " An interface only describes behavior. It has no state." Also: But if we try to do the same with an interface, the picture is different. We can try to add variables to it: 
public interface CanFly {

   String species = new String();
   int age = 10;

   public void fly();
}

public interface CanFly {

   private String species = new String(); // Error
   private int age = 10; // Another error

   public void fly();
}
We can't even declare private variables inside an interface. Why? Because the private modifier was created to hide the implementation from the user. And an interface has no implementation inside it: there isn't anything to hide. An interface only describes behavior. Accordingly, we can't implement getters and setters inside an interface. This is the nature of interfaces: they are needed to work with behavior, not state. Ok, Isn't the code in the WeatherType.java implementation? Once again, I know I'm stupid, but this is.... in the article you say "this cannot be done" and here I see it done...What?!