Question

How can I bind together two Java classes?

I’m programming a 3D opengl program where I have several different classes. I know how to bind classes to the main class, but I can’t bind two separate classes together. I have a class Terrain where I have a list containing some data about the terrain. Then I need that data in another class called Figure which isn’t the main class. I have tried to bind these classes together like this:

In Terrain class I have:

Figure fig;

public void bindClasses(Figure fg) {
   fig = fg;
}

And then in the Figure class I have:

Terrain ter;

public void bindTerrain(Terrain tr) {
   ter = tr;
}

And then in both classes I call those functions. Shouldn’t that bind them and their variables? At least that’s how I have bound my classes with the main class.

Answer 1

Just to start off with terminology. A class is a blueprint that tells you how an instantiated object is – the moment you write new Figure(), you’ve created an instance, an object of the class Figure (often you have several objects of one class). So when you are “binding” above, you are not actually binding classes, you are binding objects.

The above code is fine. By convention, you often write that sort of things with setters, it’s not necessary to call them that, but a very common pattern is:

public class Terrain {

    Figure fig

    public void setFigure(Figure fig) {
       this.fig = fig;
    }

}

public class Figure {

    Terrain ter

    public void setTerrain(Terrain ter) {
       this.ter = ter;
    }

}

To associate the two together you would in your main class do, which, as you see, is pretty much exactly what you would have already, just using the conventional names.

public void init() {

   Terrain ter = new Terrain();   // create object of class Terrain
   Figure fig = new Figure();     // create object of class Figure

   ter.setFigure(fig);
   fig.setTerrain(ter);

}

If there’s a relationship between the two objects, for instance, you can’t create a figure without having a terrain to put it in. Which would make terrain almost “own” figure. You could indicate this by using the constructor on figure.

public class Figure {

    Terrain ter

    // constructor requires an instance of Terrain, 
    // since the figure must always be placed in a terrain
    public Figure(Terrain ter) {

       this.ter = ter;

       // let terrain know this is the main figure.
       ter.setFigure(this);          
    }

}

Now your init code would instead look:

public void init() {

   Terrain ter = new Terrain();   // create object of class Terrain
   Figure fig = new Figure(ter);  // create object of class Figure in the terrain

   // no setters needed, since figure constructor sets up
   // the relationship.

}