To your Ball class you can add getX(), getY(), and getRadius() methods.
ArrayList<Ball> balls = new ArrayList<Ball>(); ballPositionX = balls.get(0).getX(); // gets the first ball's x position ballPositionY = balls.get(0).getY(); //gets the first ball's y position ballRadius = balls.get(0).getRadius(); //gets the first ball's radius
Then, circle collision detection is easy. Take two balls. Grab the x and y positions (centers) of both of them. If those positions are within (<) ball1.radius + ball2.radius then they are colliding.