xcoord = (Ball)balls.get(i).x;

or you might have to put brackets around the first bit....

((Ball)balls.get(i)).x  ... see which one works...

at the moment im just trying to print it

println((Circle)balls.get(2).x)


my class is called Circle, array list is called balls

i get the error

x cannot be resolved or is not a field

have also tried using _x and putting another set of brackets around it

If i is the index number in the arraylist of the ball you want to look at, then balls.get(i) is how you get the ith value of the arraylist. But when the list is a list of objects, not primitives, you have to say what you are getting out of there by "casting" to a Ball object.
Hence (Ball)balls.get(i).
You then want to get the x value of this.... so you have:

((Ball)balls.get(i)).x

You can break this down into more steps if you want.....

Ball selectedball;
selectedball = (Ball)balls.get(i);
xcoord = selectedball.x;

I'm just trying to condense it down into one line in the original formulation..... hope this helps.  

Make sure that x and y are parameters of your class :

Code:

class Ball {
  
  // parameters :
  float x, y;
  
  // constructor :
  Ball(float _x, float _y) {
    this.x = _x;
    this.y = _y;
  }
  
} 

Great....

One thing I learned (from Martin Schneider on openprocessing) was to create a helper method:

Circle getball(int i)
{
 return (Circle)balls.get(i);
}

which means you can then just use getball(i).x  and so on.... saves typing all that stuff every time.

Yes...have a method in the Circle class called reverse and then you would call ((Circle)balls.get(i)).reverse(); Note you need the brackets which indicate it is a method and not a field(variable).

Your condition you are looking for .... I would use the dist function...

if ( dist(x1, y1, x2, y2)<diameter1+diameter2)) - generalised (replace variables with your  own).

Of course, just reversing the balls' direction will look a bit weird.... What you really want is a collide(int i, int j) method in your main class that is called when you have a collision between ball i and ball j and calculates the speeds after the collision by conservation of momentum in the x and y directions.

The method could calculate the speeds of the balls and then set them.
Good luck...look forward to seeing it in action!

EDIT - you have to use conservation of kinetic energy, not momentum. BUt you can just use the formulas here, under "elastic collisions" http://en.wikipedia.org/wiki/Momentum#In_one_dimension
I always meant to do this myself for a container of particles but never got around to it....