@Override
protected void paintComponent(Graphics g)
{
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g;
    for(Ball ball : this.balls)
    {
        ball.draw(g2d);
    }
    g2d.dispose();
}

@Override
public void run()
{
    while(true)
    {
        try
        {
            Thread.sleep((long) (1000.0/60.0));
        } catch (InterruptedException e)
        {
            e.printStackTrace();
        }
        repaint(); // This lags when moving mouse
        // while the code below does not
        // Graphics g = getGraphics();
        // Graphics2D g2d = (Graphics2D) g;
        // for(Ball ball : this.balls)
        // {
        //     ball.draw(g2d);
        // }
    }
}