Codigo calculadora

package tcc;

import java.awt.Font;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import java.awt.event.MouseAdapter;

import java.awt.event.MouseEvent;

import javax.swing.DefaultComboBoxModel;

import javax.swing.ImageIcon;

import javax.swing.JButton;

import javax.swing.JComboBox;

import javax.swing.JFrame;

import javax.swing.JLabel;

import javax.swing.JPanel;

import javax.swing.JScrollPane;

import javax.swing.JTextField;

import javax.swing.border.EmptyBorder;

public class MenuUsuario extends JFrame {

        private static final long serialVersionUID = 1L;

        private JPanel contentPane;

        private JTextField jtfpesquisanome;

        public static ListagemUsuarios lista=new ListagemUsuarios();

        private JScrollPane painel=new JScrollPane(lista.table);

        Usuário j = new Usuário();

        public MenuUsuario() {

                setResizable(false);

                lista.defineRenderers();

                lista.preencheTabela(j.listagemUsuario());

                setTitle("MENU USUARIO");

                setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

                setLocation(100,140);

                setSize(649,346);

                contentPane = new JPanel();

                contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));

                setContentPane(contentPane);

        //        lista.table.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);

                

                painel.setBounds(12, 135, 621, 162);

                getContentPane().add(painel);

                JButton jbaddCliente = new JButton("");

                jbaddCliente.setIcon(new ImageIcon(MenuUsuario.class.getResource("/imagens/add.png")));

                jbaddCliente.addMouseListener(new MouseAdapter() {

                        @Override

                        public void mouseEntered(MouseEvent arg0) {

                                jbaddCliente.setToolTipText("

ADICIONAR CLIENTE

");

                        }

                });

                jbaddCliente.setBounds(12, 13, 50, 50);

                jbaddCliente.addActionListener(new ActionListener() {

                        public void actionPerformed(ActionEvent arg0) {

                                

                                CadastroUsuario cadcli=new CadastroUsuario();

                                cadcli.setVisible(true);

                                

                        }

                });

                

                contentPane.setLayout(null);

                contentPane.add(jbaddCliente);

                

                JButton jbeditar = new JButton("");

                jbeditar.setIcon(new ImageIcon(MenuUsuario.class.getResource("/imagens/editar.png")));

                jbeditar.addMouseListener(new MouseAdapter() {

                        @Override

                        public void mouseEntered(MouseEvent e) {

                                jbeditar.setToolTipText("

EDITAR CADASTRO USUÁRIO

");

                        }

                });

                jbeditar.setBounds(74, 13, 50, 50);

                contentPane.add(jbeditar);

                

                JButton jbexcluir = new JButton("");

                jbexcluir.setIcon(new ImageIcon(MenuUsuario.class.getResource("/imagens/excluir.png")));