Erros com JTextField

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 

class Gui extends JFrame implements ItemListener { 

private JLabel lblfornecedora, lblmodelo, lblforma, lblop1, lblcarro,lblpreco, lblop; 
private JLabel lblvvenda, lblvprest, lblvdesc, lblvtotal; 
private JTextField txtCarro, txtpreco, txtop, txtvvenda, txtvprest,txtvdesc, txtvtotal; 
private JButton btnCalcular, btnLimpar, btnSair; 
private JComboBox cbFornecedora; 
private JPanel pnlNorte1, pnlSul1, pnlCentro1, pnl1, pnl2, pnlNorte2,pnlSul2; 
private Container C; 
private JRadioButton radio1, radio2, radio3, radio4; 
private JCheckBox check1, check2, check3, check4; 
public JList list1; 
DefaultListModel dfl; 
private ButtonGroup radiogrupo; 


String Volkswagen[] = {"Gol 1000", "Gol CL 1.6", "Golf GL 1.6", 
"Parati 1.0 Mi 16v", "Apollo GL", "Logus CL 1.8", "Pollo Classic", 
"Quantum CL 1.8", "Quantum GL 2.0"}; 

String Fiat[] = {"Palio ED", "Elba Weekend", "Tipo 1.6", "Marea"}; 

String Chevrolet[] = {"Corsa Wind 1.0","Corsa Sedan GL","Kadett Sl 1.8","Kadett Sl 2.0","Astra GL 1.8","Astra GL 2.0"}; 

String Ford[] = {"Ka 1.0","Ka 1.3","Escort L 1.8","Fiesta 1.0"}; 

Gui() { 
String Fornecedoras[] = {"Selecione...","Volkswagen", "Chevrolet", "Fiat", "Ford"}; 
String Fornec[] = {"Volkswagen", "Chevrolet", "Fiat", "Ford"}; 
String Chevrolet[] = {"Corsa Wind 1.0","Corsa Sedan GL","Kadett Sl 1.8","Kadett Sl 2.0","Astra GL 1.8","Astra GL 2.0"}; 

pnlNorte1 = new JPanel(new GridLayout(2, 2)); 
pnlNorte2 = new JPanel(new GridLayout(7, 2)); 

pnlSul1 = new JPanel(new GridLayout(5, 1)); 
pnlCentro1 = new JPanel(new GridLayout(6, 1)); 
setTitle("Catálogo de Vendas de Carros"); 
setSize(700, 400); 
C = getContentPane(); 
C.setLayout(new GridLayout(1, 2)); 

pnl1 = new JPanel(); 
pnl1.setLayout(new BorderLayout()); 
pnl1.setBorder(new javax.swing.border.TitledBorder("Dados do Carro")); 
pnl1.add(pnlNorte1, BorderLayout.NORTH); 
pnl1.add(pnlSul1, BorderLayout.SOUTH); 
pnl1.add(pnlCentro1, BorderLayout.CENTER); 

pnlNorte1.add(lblfornecedora = new JLabel(" Fornecedora")); 
pnlNorte1.add(lblmodelo = new JLabel(" Modelo")); 
pnlNorte1.add(cbFornecedora = new JComboBox(Fornecedoras)); 
/* 
* Inicio da manipulacao do Jlist 
*/ 
dfl = new DefaultListModel(); 
pnlNorte1.add(list1 = new JList(dfl)); 
// Adicionando os itens iniciais 
for (int i = 0; i < Fornec.length; i++) 
dfl.add(i, Fornec[i]); 

list1.setVisibleRowCount(3); //define o numero de linhas visíveis 
pnlNorte1.add(new JScrollPane(list1)); 
//adiciona uma barra de rolagem ao list 
list1.setSelectionMode(ListSelectionModel.SINGLE_SELECTION); 
//o usuário seleciona apenas uma opção 

cbFornecedora.setMaximumRowCount(3); 
cbFornecedora.addItemListener(this); 

radiogrupo = new ButtonGroup(); 
pnlSul1.add(lblforma = new JLabel("Forma de Pagamento")); 
pnlSul1.add(radio1 = new JRadioButton("a vista")); 
pnlSul1.add(radio2 = new JRadioButton("a prazo - 12 Meses")); 
pnlSul1.add(radio3 = new JRadioButton("a prazo - 24 Meses")); 
pnlSul1.add(radio4 = new JRadioButton("a prazo - 36 Meses")); 

radiogrupo.add(radio1); 
radiogrupo.add(radio2); 
radiogrupo.add(radio3); 
radiogrupo.add(radio4); 

pnlCentro1.add(lblop1 = new JLabel("Opcionais")); 
pnlCentro1.add(check1 = new JCheckBox("Direção Hidráulica - R$ 2.700,00")); 
pnlCentro1.add(check2 = new JCheckBox("Air Bag - R$ 3.200,00")); 
pnlCentro1.add(check3 = new JCheckBox("4 Portas - R$ 850,00")); 
pnlCentro1.add(check4 = new JCheckBox("Ar condicionado - R$ 1.850,00")); 

//check1.addItemListener(this); 

C.add(pnl1); 

pnl2 = new JPanel(); 
pnlSul2 = new JPanel(new FlowLayout()); 
pnl2.setLayout(new BorderLayout()); 
pnl2.setBorder(new javax.swing.border.TitledBorder("Dados da Venda")); 
pnl2.add(pnlNorte2, BorderLayout.NORTH); 
pnl2.add(pnlSul2, BorderLayout.SOUTH); 

pnlNorte2.add(lblcarro = new JLabel("Carro::")); 
pnlNorte2.add(txtCarro = new JTextField()); 
pnlNorte2.add(lblpreco = new JLabel("Preço::")); 
pnlNorte2.add(txtpreco = new JTextField()); 
pnlNorte2.add(lblop = new JLabel("Opcionais::")); 
pnlNorte2.add(txtop = new JTextField()); 
pnlNorte2.add(lblvvenda = new JLabel("Valor da Venda::")); 
pnlNorte2.add(txtvvenda = new JTextField()); 
pnlNorte2.add(lblvprest = new JLabel("Valor da Prestação::")); 
pnlNorte2.add(txtvprest = new JTextField()); 
pnlNorte2.add(lblvdesc = new JLabel("Valor do Desconto::")); 
pnlNorte2.add(txtvdesc = new JTextField()); 
pnlNorte2.add(lblvtotal = new JLabel("Valor Total::")); 
pnlNorte2.add(txtvtotal = new JTextField()); 

pnlSul2.add(btnCalcular = new JButton("Calcular")); 
pnlSul2.add(btnLimpar = new JButton("Limpar")); 
pnlSul2.add(btnSair = new JButton("Sair")); 
C.add(pnl2); 
pnl2.add(pnlNorte2); 

} 


public void itemStateChanged(ItemEvent event) { 
if (event.getStateChange() == ItemEvent.SELECTED) { 
if (cbFornecedora.getSelectedIndex() == 1) { 

dfl.clear(); 
for (int i = 0; i < Volkswagen.length; i++) 
dfl.add(i, Volkswagen[i]); //Substitui aqui os itens da list 

} 
else if (cbFornecedora.getSelectedIndex() == 2) { 

dfl.clear(); 
for (int i = 0; i < Chevrolet.length; i++) 
dfl.add(i, Chevrolet[i]); 
} 

else if(cbFornecedora.getSelectedIndex() ==3) { 
dfl.clear(); 
for(int i=0;i<Fiat.length;i++) 
dfl.add(i,Fiat[i]); 
} 

else { 
dfl.clear(); 
for(int i=0;i<Ford.length;i++) 
dfl.add(i,Ford[i]); 
} 
} 

}//fim tratamento de evento 


/*list1.addListSelectionListener( 

new ListSelectionListener() 
{ 
public void valueChanged(ListSelectionEvent event) 
{ 
txtCarro.setText(list1.getSelectedItem()); 
} 

} 

) 

*/ 
public static void main(String args[]) 
{ 
Gui DlgGui = new Gui(); 
DlgGui.show(); 
}//Fim main 

}//Fim da Classe GUI