Observer pattern on GWT

Hey there! I'm relatively new to both GWT and java programming (or OOP for that matter), so apologies for the beginner questions/mistakes in advance. I've been trying to create some kind of observer pattern, but the development mode console keeps dropping error messages and sadly, they're far from helpful.

So here's what I'm trying to achieve: - I've got the model that consists of the class Country, and stores a value called Influence. - The view is the class called CountryDisplay. It's a GWT widget that should always display the current influence of a given country.

public class Country {
   private int influece;
   private CountryDisplay display;

   public Country() {
      influence = 0;
   }
   public void setDisplay(CountryDisplay display) //...
   public int getInfluence() //...
   public void setInfluence(int value) {
      influence = value;
      display.update();
   }
}
public class CountryDisplay {

   private Country country;

   public CountryDisplay (Country country) {
      //GWT widget creating stuff
      this.country = country;
   }
   public void update() {
      //InfluenceCounter is a simple Label
      InfluenceCounter.setText(Integer.toString(country.getInfluence()));
   }
}

Then in the EntryPoint class I do something like this:

Country italy = new Country(); 
CountryDisplay italyDisplay = new CountryDisplay(italy);
italy.setDisplay(italyDisplay);
RootPanel.get("nameFieldContainer").add(italyDisplay);
italy.setInfluence(3);

The development console indicated that it had a problem with the line "display.update();" in class Country. My first guess was that the problem was that the display was not initiated, so I created an interface for it, and in the Country constructor I created an empty, new displa开发者_如何学编程y, that would later be overwritten.

   public Country() {
      influence = 0;
      display = new DisplayInterface() {
          public void update() {}
      }
   }  

But I had no luck this way either. I guess this kind of cross-referencing is not allowed? I mean that the view has the model as a variable and vice versa. When calling a method on the view individually (like:

italy.setInfluence(3);
italyDisplay.displayTheCurrentValue();

) it works, so the problem is definitely in the observer logic.

If I understand correctly, your are trying to "bind" user interface elements (your view class CountryDisplay) to data (the model class Country). "Bind" in the sense that if you change the model data (for example, call italy.setInfluence(10)) then the view would automatically update itself to reflect the change. And if your view provided an editor, you want the "binding" also to work in the other direction.

There are several frameworks out there that achieve this, see for example the post Best data binding solution for GWT. I have used GWT Pectin and there is the GWT Editors framework (which I have not yet used myself as it is relatively new).

Looking at your code, I feel you might want to more clearly separate the model from the view: your model class (Country) should not know about the view class, that is, it should not store a reference to CountryDisplay.