public void itemStateChanged(ItemEvent e) {
   int evento = e.getStateChange();
   if (evento == ItemEvent.SELECTED) {
     String nivel = comboNivel.getSelectedItem().toString();
     Fachada fachada = Fachada.getInstancia();
     List<Aula> lista = fachada.cadastroAula().buscarNivel(nivel);
     tabela.montarTabela(lista);
   }
 }
  public TelaManterAulas() {
    setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    setBounds(100, 100, 658, 442);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);

    JPanel panel = new JPanel();
    panel.setBackground(Color.WHITE);
    panel.setBounds(10, 11, 630, 389);
    contentPane.add(panel);
    panel.setLayout(null);

    JLabel lblListagemDeAulas = new JLabel("Manter de Aulas");
    lblListagemDeAulas.setFont(new Font("Tahoma", Font.PLAIN, 25));
    lblListagemDeAulas.setBounds(196, 11, 190, 50);
    panel.add(lblListagemDeAulas);

    JLabel lblNvel = new JLabel("N\u00EDvel:");
    lblNvel.setBounds(35, 93, 46, 14);
    panel.add(lblNvel);

    comboNivel = new JComboBox<String>();
    comboNivel.setBounds(74, 90, 80, 20);
    panel.add(comboNivel);
    carregarCombo();
    comboNivel.addItemListener(this);

    JScrollPane scrollPane = new JScrollPane();
    scrollPane.setBounds(35, 138, 555, 187);
    panel.add(scrollPane);

    tabela = new Tabela(new String[] {"Descrição", "Nível", "Material"});
    tabela.getColumnModel().getColumn(0).setResizable(false);
    tabela.getColumnModel().getColumn(0).setPreferredWidth(300);
    tabela.getColumnModel().getColumn(1).setResizable(false);
    tabela.getColumnModel().getColumn(1).setPreferredWidth(50);
    tabela.getColumnModel().getColumn(2).setResizable(false);
    tabela.getColumnModel().getColumn(2).setPreferredWidth(300);
    tabela.montarTabela(Fachada.getInstancia().cadastroAula().listarAulas());
    scrollPane.setViewportView(tabela);

    btnVisualizar = new JButton("Visualizar");
    btnVisualizar.setBounds(294, 336, 98, 23);
    panel.add(btnVisualizar);

    btnRemover = new JButton("Remover");
    btnRemover.addActionListener(this);
    btnRemover.setBounds(402, 336, 89, 23);
    panel.add(btnRemover);

    btnEditar = new JButton("Editar");
    btnEditar.setBounds(501, 336, 89, 23);
    panel.add(btnEditar);
  }
  private void remover() {

    int linha = tabela.getSelectedRow();
    if (linha != -1) {
      Object[] options = {"OK", "Cancelar"};
      int resposta =
          JOptionPane.showOptionDialog(
              this,
              "Tem certeza que deseja remover?",
              "Alerta !!",
              JOptionPane.DEFAULT_OPTION,
              JOptionPane.WARNING_MESSAGE,
              null,
              options,
              options[0]);
      if (resposta == 0) {
        Aula aula = (Aula) tabela.getModel().getValueAt(linha, 0);
        Fachada fachada = Fachada.getInstancia();
        fachada.cadastroAula().removerAula(aula);
        tabela.montarTabela(Fachada.getInstancia().cadastroAula().listarAulas());
      }
    }
  }