Jtable y Jcomboox

Hola soy nuevo en esta comunidad, recien empiezo a programar y tengo una duda. les pongo mi codigo y les explico que sucede.
package venta;

import java.awt.EventQueue;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.GroupLayout;
import javax.swing.GroupLayout.Alignment;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.table.DefaultTableModel;
import javax.swing.table.TableModel;
import javax.swing.LayoutStyle.ComponentPlacement;
import javax.swing.JLabel;
import javax.swing.SwingConstants;
import java.awt.Font;
import javax.swing.JTextField;
import javax.swing.JTable;
import javax.swing.JButton;
import javax.swing.border.TitledBorder;
import java.awt.Color;
import javax.swing.border.BevelBorder;
import java.awt.SystemColor;
import javax.swing.JSeparator;

///////////////////////////////////////////////////////
public class TeatroVentana extends JFrame {
private JComboBox combo1;
private JComboBox combo2;
private JTextField cantidad;
private DefaultTableModel modelo = new DefaultTableModel();
private JTable table = new JTable();
private JTable tablita;
private JTextField ganancia;

///////////////////////////////////////////////////////////////
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
TeatroVentana frame = new TeatroVentana();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}

///////////////////////////////////////////
public TeatroVentana() {
setResizable(false);
setTitle("Teatro");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 465, 382);
JPanel ventana = new JPanel();
ventana.setBackground(SystemColor.inactiveCaptionBorder);
ventana.setBorder(new BevelBorder(BevelBorder.LOWERED, new Color(0, 0, 0), null, null, null));
setContentPane(ventana);

combo1 = new JComboBox();
combo1.setFont(new Font("Arial", Font.PLAIN, 14));
combo1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent arg0) {
try {
//
rellena(combo1.getSelectedIndex());
} catch (Exception e) {
e.printStackTrace();
}
}
});

combo2 = new JComboBox();
combo2.setFont(new Font("Arial", Font.PLAIN, 14));

JLabel lblBoletos = new JLabel("Boletos");
lblBoletos.setFont(new Font("Perpetua", Font.BOLD | Font.ITALIC, 25));
lblBoletos.setHorizontalAlignment(SwingConstants.CENTER);

cantidad = new JTextField();
cantidad.setHorizontalAlignment(SwingConstants.CENTER);
cantidad.setColumns(10);

JLabel lblCantidad = new JLabel("Cantidad:");
lblCantidad.setFont(new Font("Arial", Font.PLAIN, 14));

JButton btnAceptar = new JButton("Aceptar");
btnAceptar.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
hacerCompra();
}
});

JLabel lblAsientos = new JLabel("Asientos");
lblAsientos.setHorizontalAlignment(SwingConstants.CENTER);
lblAsientos.setFont(new Font("Perpetua", Font.BOLD | Font.ITALIC, 25));

JButton btnMapa = new JButton("Mapa");

tablita = new JTable();
tablita.setBorder(new BevelBorder(BevelBorder.RAISED, new Color(0, 0, 0), null, null, null));
tablita.setModel(modelo);
tablita.setBounds(new Rectangle(10, 60, 510, 370));
modelo.setColumnCount(4);
modelo.setRowCount(5);

modelo.setValueAt( 39 ,1 ,1 );
modelo.setValueAt( 38 ,2 ,1 );
modelo.setValueAt( 72 ,1 ,2 );
modelo.setValueAt( 48 ,2 ,2 );
modelo.setValueAt( 121,3 ,2 );
modelo.setValueAt( 153,1 ,3 );
modelo.setValueAt( 142,2 ,3 );
modelo.setValueAt( 15 ,3 ,3 );
modelo.setValueAt( 11 ,4 ,3 );

ganancia = new JTextField();
ganancia.setEditable(false);
ganancia.setColumns(10);

JLabel lblGanancia = new JLabel("Ganancia:");
lblGanancia.setFont(new Font("Arial", Font.PLAIN, 14));

GroupLayout gl_ventana = new GroupLayout(ventana);
gl_ventana.setHorizontalGroup(
gl_ventana.createParallelGroup(Alignment.TRAILING)
.addGroup(gl_ventana.createSequentialGroup()
.addContainerGap()
.addGroup(gl_ventana.createParallelGroup(Alignment.TRAILING)
.addComponent(lblBoletos, Alignment.LEADING, GroupLayout.DEFAULT_SIZE, 208, Short.MAX_VALUE)
.addGroup(gl_ventana.createSequentialGroup()
.addComponent(lblCantidad, GroupLayout.DEFAULT_SIZE, 137, Short.MAX_VALUE)
.addPreferredGap(ComponentPlacement.RELATED)
.addComponent(cantidad, GroupLayout.PREFERRED_SIZE, 66, GroupLayout.PREFERRED_SIZE))
.addComponent(combo2, 0, 208, Short.MAX_VALUE)
.addComponent(combo1, Alignment.LEADING, 0, 208, Short.MAX_VALUE)
.addComponent(btnAceptar, GroupLayout.PREFERRED_SIZE, 99, GroupLayout.PREFERRED_SIZE))
.addGap(20)
.addGroup(gl_ventana.createParallelGroup(Alignment.TRAILING)
.addGroup(gl_ventana.createParallelGroup(Alignment.LEADING)
.addComponent(btnMapa)
.addComponent(tablita, GroupLayout.PREFERRED_SIZE, 203, GroupLayout.PREFERRED_SIZE)
.addGroup(gl_ventana.createSequentialGroup()
.addPreferredGap(ComponentPlacement.RELATED)
.addComponent(lblAsientos, GroupLayout.PREFERRED_SIZE, 203, GroupLayout.PREFERRED_SIZE)))
.addGroup(gl_ventana.createSequentialGroup()
.addComponent(lblGanancia)
.addPreferredGap(ComponentPlacement.RELATED)
.addComponent(ganancia, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE)))
.addContainerGap())
);
gl_ventana.setVerticalGroup(
gl_ventana.createParallelGroup(Alignment.LEADING)
.addGroup(gl_ventana.createSequentialGroup()
.addContainerGap()
.addGroup(gl_ventana.createParallelGroup(Alignment.BASELINE)
.addComponent(lblBoletos, GroupLayout.PREFERRED_SIZE, 22, GroupLayout.PREFERRED_SIZE)
.addComponent(lblAsientos))
.addPreferredGap(ComponentPlacement.RELATED)
.addGroup(gl_ventana.createParallelGroup(Alignment.LEADING)
.addGroup(gl_ventana.createSequentialGroup()
.addComponent(tablita, GroupLayout.PREFERRED_SIZE, 80, GroupLayout.PREFERRED_SIZE)
.addPreferredGap(ComponentPlacement.RELATED)
.addComponent(btnMapa))
.addGroup(gl_ventana.createSequentialGroup()
.addComponent(combo1, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE)
.addPreferredGap(ComponentPlacement.RELATED)
.addComponent(combo2, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE)
.addPreferredGap(ComponentPlacement.RELATED)
.addGroup(gl_ventana.createParallelGroup(Alignment.BASELINE)
.addComponent(cantidad, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE)
.addComponent(lblCantidad))
.addPreferredGap(ComponentPlacement.RELATED)
.addComponent(btnAceptar)))
.addPreferredGap(ComponentPlacement.RELATED, 151, Short.MAX_VALUE)
.addGroup(gl_ventana.createParallelGroup(Alignment.BASELINE)
.addComponent(ganancia, GroupLayout.PREFERRED_SIZE, GroupLayout.DEFAULT_SIZE, GroupLayout.PREFERRED_SIZE)
.addComponent(lblGanancia))
.addContainerGap())
);
ventana.setLayout(gl_ventana);
init();
}
///////////////////////
void rellena(int num){
while (combo2.getModel().getSize() > 0) combo2.removeItemAt(0);
if(num==0){
combo2.addItem("Seleccione la Seccion");

}else if(num==1){
combo2.addItem("Seleccione la Seccion");
combo2.addItem("Seccion 1");
combo2.addItem("Seccion 2");
}else if(num==2){
combo2.addItem("Seleccione la Seccion");
combo2.addItem("Seccion 1");
combo2.addItem("Seccion 2");
combo2.addItem("Seccion 3");
}else if(num==3){
combo2.addItem("Seleccione la Seccion");
combo2.addItem("Seccion 1");
combo2.addItem("Seccion 2");
combo2.addItem("Seccion 3");
combo2.addItem("Seccion 4");

}
}
void init(){
while (combo1.getModel().getSize() > 0) combo1.removeItemAt(0);
combo1.addItem("Seleccione la Zona");
combo1.addItem("Zona 1");
combo1.addItem("Zona 2");
combo1.addItem("Zona 3");
}
void hacerCompra(){

}
}

Bueno resulta que el programa hasta este punto me muestra una tabla con datos, y la 2da combobox me muestran secciones dependiendo primera, lo que quiero lograr es que al momento de pulsar el boton, este dependiendo si en la combobox1 esta "Zona 1" y en la combobox esta "Seccion 1" y dependiendo el la cantidad ingresada en el txtfield "compra" por ejemplo ingreso 3, y al pulsar el boton este le reste 3 al 39 que esta en modelo.setValueAt( 39 ,1 ,1 ) modificando el valor de la tabla automaticamente.