How to use an instance instead of the class itself

Question

I am building a program but have a problem.
My program uses a controller class which instantiates a World-class. In the world class has a Vector I make a Vector in which I place all the elements in the world. (I also made an Element class from which all the objects that I put into this Vector extend.)
Everything except for some methods in the controller class are declared public. I might need to use static, but I do not know how.
// CONSTRUCTOR CONTROLLER CLASS
world = new World ();  // instantiate one and only one world
world.newElement (new Beamer ("beamer", 20. , 20.));  // use a selfwritten method to add a "Star Trek"-like
                          // beamer to the world. I give it a name and position.
world.newElement (new Avatar ("avt", "beamer"));  // add an avatar which is "beamed up". so what I do is that I use the beamer for the beginposition of the avatar. However, and this is my problem, in the constructor of Avatar (see below) i need the beamer in world, but my compiler cant find world. I probably need to import my World or controller class, but even then: I do not understand how to use an instance (world is an instance. I cant use the World-class as the class does not contain the beamer element)
public class Avatar extends Element {
     public Avatar (String n, String beamer){
          name = n;
          posx = world.named("beamer").getPosx; // the selfwritten method named finds the
            // element in the Vector that goes with the name. Any ideas to make this code better are welcome too.
          posy = world.named("beamer").getPosy;
} Any ideas on how to solve this (common) problem in good coding are most welcome.

Answer 1

I think you are best off if you use static methods and static fields only in your World class. That way you don't have to instantiate it and everyone has access to its methods. This would be then completely analogous to how you access methods and fields of the well-known class System (e.g. when you use System.out.println). This also has the advantage that you are guaranteed to have only one copy of the World.
Here is an example of how your World class, Avatar class and main code would look like.
Main code: (note the use of capital W in the name World, referring to class rather than instance).
// no need to instantiate World, as we call its static methods only
World.newElement (new Beamer ("beamer", 20. , 20.));
World.newElement (new Avatar ("avt", "beamer"));  // add an avatar which is "beamed up". World class: (note the static keyword in front of the vector and the methods)
public class World {
  private static Vector v;       // static field, only one copy of it ever exists
  public static void newElement(Element e)   // static method, can call without instantiating world
     v.addElement(e);
  public static Element named(String n)  // use to look up elements
     ... some code which goes through v and finds element named "n" ...
     ... return reference to that element ...
}and finally the Avatar: (again note the use of reference to World as a class rather than instance)
public class Avatar extends Element {
     public Avatar (String n, String beamer){
          name = n;
          posx = World.named("beamer").getPosx;
          posy = World.named("beamer").getPosy;
} The above example is how I would implement it when you don't actually need instances of the World class.