Qual o problema da classe

package Exercicio8;
import javax.swing.*;
import java.awt.*;
public class Exercicio8 extends JFrame{
	
	private final int LARGURA = 600;
	private final int ALTURA = 400;
	private int altura;
	private int largura;
	private String texto;

	private JPanel principal;
	private JPanel painelVerde;
	private JPanel painelAmarelo;
	private JPanel painelLaranja;
	
	private JLabel jlRotulo1, jlRotulo2, jlRotulo3;
	
	private CardLayout layout;
	
	public Exercicio8() {
		jlRotulo1 = new JLabel("Painel Verde");
		layout = new CardLayout();
		principal = new JPanel();
		principal.setLayout(layout);
		principal.add(jlRotulo1);
		setLayout(new BorderLayout());
		add(principal);
	}
}