package bancob;
public abstract class Conta {
 
private String cliente;
private int numero;
private double saldo;
 
public Conta(String cliente, int numero, double saldo) {
this.cliente = cliente;
this.numero = numero;
this.saldo = saldo;
}
 
public String getCliente() {
return cliente;
}
 
public void setCliente(String cliente) {
this.cliente = cliente;
}
 
public int getNumero() {
return numero;
}
 
public void setNumero(int numero) {
this.numero = numero;
}
 
public double getSaldo() {
return saldo;
}
 
public void setSaldo(double saldo) {
this.saldo = saldo;
}
 
public abstract void sacar(double valor);
public abstract void depositar(double valor);
}
 
package bancob;
 
public class ContaCorrente extends Conta {
 
private double limite;
 
public ContaCorrente(double limite, String cliente, int numero, double saldo) {
super(cliente, numero, saldo);
this.limite = limite;
}
 
public double getLimite() {
return limite;
}
 
public void setLimite(double limite) {
this.limite = limite;
}
 
@Override
public void sacar(double valor) {
if(valor maior= (this.getSaldo()+limite)){
System.out.println("Sacou "+valor);
this.setSaldo(this.getSaldo()-valor);
} else {
System.out.println("Saldo insuficiente");
}
}
 
@Override
public void depositar(double valor) {
throw new UnsupportedOperationException("Not supported yet.");
}
 
JTable é o objeto da API swing responsável por exibir uma tabela, imagina uma planilha do excel.
 
O JTAble não deve ficar responsável por absolutamente nada do que diz respeito a gestão dos dados que serão exibidos o componente caberá a função de definir onde no JFrame será exibida a tabela, todo o resto deve ser programado em uma classe auxiliar que irá extender a classe AbstractTableModel.
 
Nesta classe você irá definir a quantidade de colunas, de que maneira cada uma delas será exibida, quais serão os dados e de que maneira eles (os dados serão manipulados).
 
Segue abaixo o código completo da aplicação funcionando.

package bancob;
 
import java.text.DecimalFormatSymbols;
import java.util.Locale;
import java.util.Vector;
import javax.swing.JOptionPane;
import javax.swing.table.AbstractTableModel;
 
public class ListarContaCorrete extends javax.swing.JDialog {
 
    ModeloTabelaContas modelo = new ModeloTabelaContas();
    int linhaselecionada;
    ContaCorrente contaSelecionada;
 
    public ListarContaCorrete(java.awt.Frame parent, boolean modal) {
        super(parent, modal);
 
        initComponents();
 
        tbContas.setModel(modelo);
    }
 
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {
 
        jScrollPane1 = new javax.swing.JScrollPane();
        tbContas = new javax.swing.JTable();
        txtnome = new javax.swing.JTextField();
        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        jLabel3 = new javax.swing.JLabel();
        jButton1 = new javax.swing.JButton();
        jButton2 = new javax.swing.JButton();
        txtidconta = new javax.swing.JLabel();
        jButton3 = new javax.swing.JButton();
        jButton4 = new javax.swing.JButton();
        txtsaldo = new javax.swing.JFormattedTextField();
        txtlimite = new javax.swing.JFormattedTextField();
 
        setDefaultCloseOperation(javax.swing.WindowConstants.DISPOSE_ON_CLOSE);
 
        jScrollPane1.setViewportView(tbContas);
        tbContas.getAccessibleContext().setAccessibleName("");
 
        jLabel1.setText("Nome:");
 
        jLabel2.setText("Saldo:");
 
        jLabel3.setText("Limite:");
 
        jButton1.setText("Salvar");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });
 
        jButton2.setText("Carregar");
        jButton2.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton2ActionPerformed(evt);
            }
        });
 
        txtidconta.setText("Conta: não selecionada");
 
        jButton3.setText("Remover");
        jButton3.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton3ActionPerformed(evt);
            }
        });
 
        jButton4.setText("Limpar");
        jButton4.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton4ActionPerformed(evt);
            }
        });
 
        txtsaldo.setFormatterFactory(new javax.swing.text.DefaultFormatterFactory(new javax.swing.text.NumberFormatter(new java.text.DecimalFormat("#0.00"))));
 
        txtlimite.setFormatterFactory(new javax.swing.text.DefaultFormatterFactory(new javax.swing.text.NumberFormatter(new java.text.DecimalFormat("#0.00"))));
 
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jLabel1)
                        .addGap(10, 10, 10)
                        .addComponent(txtnome, javax.swing.GroupLayout.PREFERRED_SIZE, 190, javax.swing.GroupLayout.PREFERRED_SIZE))
                    .addComponent(txtidconta)
                    .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jLabel2)
                            .addComponent(jLabel3))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                            .addComponent(txtsaldo, javax.swing.GroupLayout.DEFAULT_SIZE, 75, Short.MAX_VALUE)
                            .addComponent(txtlimite)))
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addComponent(jButton1)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jButton4)
                        .addGap(14, 14, 14)))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 12, Short.MAX_VALUE)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jButton2)
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addComponent(jButton3))
                    .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 436, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(txtidconta)
                    .addComponent(jButton2)
                    .addComponent(jButton3))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 252, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(txtnome, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jLabel1))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jLabel2)
                            .addComponent(txtsaldo, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jLabel3)
                            .addComponent(txtlimite, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                            .addComponent(jButton1)
                            .addComponent(jButton4))))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
 
        pack();
    }// </editor-fold>
 
    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
 
        String nome = txtnome.getText();
        if(nome.equals("")){
            JOptionPane.showMessageDialog(this, "Nome obrigatório");
            txtnome.requestFocus();
            return;
        }
 
        double limite = 0;
        if(!txtlimite.getText().equals("")){
            limite = Double.parseDouble(txtlimite.getText().replace(",", "."));
        }
 
        double saldo = 0;
        if(!txtsaldo.getText().equals("")){
            saldo = Double.parseDouble(txtsaldo.getText().replace(",", "."));
        }
 
        ContaCorrente c = new ContaCorrente(limite, nome, 10, saldo);
 
        if(contaSelecionada == null){
            modelo.insertData(c);
        } else {
            modelo.updateData(c, linhaselecionada);
            linhaselecionada = -1;
            contaSelecionada = null;
        }
 
    }                                        
 
    private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {                                         
 
        linhaselecionada = tbContas.getSelectedRow();
 
        if(linhaselecionada == -1){
            JOptionPane.showMessageDialog(null, "Selecione uma linha");
        }
 
        contaSelecionada = modelo.get(linhaselecionada);
 
        txtidconta.setText("Conta: "+Integer.toString(linhaselecionada));
        txtnome.setText(contaSelecionada.getCliente());
        txtsaldo.setText(Double.toString(contaSelecionada.getSaldo()));
        txtlimite.setText(Double.toString(contaSelecionada.getLimite()));
 
    }                                        
 
    private void jButton3ActionPerformed(java.awt.event.ActionEvent evt) {                                         
 
        int opcao = JOptionPane.showConfirmDialog(this, "Deseja Remover a Conta?"); 
 
        if(opcao == JOptionPane.YES_OPTION){
            if(tbContas.getSelectedRow() >= 0){
                modelo.delete(tbContas.getSelectedRow());
                JOptionPane.showMessageDialog(this, "Conta removida com sucesso");
            }
        }
    }                                        
 
    private void jButton4ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        linhaselecionada = -1;
        contaSelecionada = null;
 
        txtidconta.setText("Conta: não sellecionada");
        txtnome.setText("");
        txtlimite.setText("");
        txtsaldo.setText("");
 
    }                                        
 
    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(ListarContaCorrete.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(ListarContaCorrete.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(ListarContaCorrete.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(ListarContaCorrete.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>
 
        /* Create and display the dialog */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                ListarContaCorrete dialog = new ListarContaCorrete(new javax.swing.JFrame(), true);
                dialog.addWindowListener(new java.awt.event.WindowAdapter() {
                    @Override
                    public void windowClosing(java.awt.event.WindowEvent e) {
                        System.exit(0);
                    }
                });
                dialog.setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify
    private javax.swing.JButton jButton1;
    private javax.swing.JButton jButton2;
    private javax.swing.JButton jButton3;
    private javax.swing.JButton jButton4;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JTable tbContas;
    private javax.swing.JLabel txtidconta;
    private javax.swing.JFormattedTextField txtlimite;
    private javax.swing.JTextField txtnome;
    private javax.swing.JFormattedTextField txtsaldo;
    // End of variables declaration
}
 
class ModeloTabelaContas extends AbstractTableModel {
 
    private static final int COL_NOME = 0;
    private static final int COL_SALDO = 1;
    private static final int COL_LIMITE = 2;
 
    private String[] columnNames = {"Nome","Saldo","Limite"};
 
    private Vector contas = new Vector();
 
    @Override
    public int getRowCount() {
        return contas.size();
    }
 
    @Override
    public int getColumnCount() {
        return columnNames.length;
    }
 
    @Override
    public String getColumnName(int col){
        return columnNames[col];
    }
 
    @Override
    public Object getValueAt(int rowIndex, int columnIndex) {
 
        ContaCorrente c = (ContaCorrente) contas.get(rowIndex);
 
        switch(columnIndex){
            case COL_SALDO:
                return c.getCliente();
            case COL_LIMITE:
                return c.getSaldo();
            default:
                return c.getLimite();
        }
    }
 
    public void insertData(ContaCorrente c){
        contas.add(c);
        fireTableDataChanged();
    }
 
    public void updateData(ContaCorrente c, int pos){
        contas.remove(pos);
        contas.add(pos,c);
        fireTableDataChanged();
    }
 
    public ContaCorrente get(int i){
        return (ContaCorrente) contas.get(i);
    }
 
    public void delete(int i){
        contas.remove(i);
        fireTableDataChanged();
    }
}