Timer error

import javax.swing.JComponent;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.util.ArrayList;

public CarSet extends JComponent
{	
	private ArrayList<Car> carList;
	public CarSet()
	{
		carList = new ArrayList<Car>();
	}
	public void addCar(Car car)
	{
		carList.add(car);
	}
	public void paintComponent(Graphics g)
	{
		Graphics2D g2 = (Graphics2D) g;
		for ( Car a: carList)
		{
			a.draw(g2);
		}
	}
	public void moveBy(dx,dy)
	{
		for (int i = 0; i<carList.length() ; i++ )
		{
			if (i%2==0)
			{
				carList.get(i).translate(dx,dy);
			}
			else carList.get(i).translate(-dx,dy);
		}
	}
}