@Override
 public void adicionar(Conta c) {
   try {
     Statement st;
     st = conexao.getConnection().createStatement();
     String sql = "insert into contas values (" + c.getNumero() + "," + c.getSaldo() + ")";
     st.executeUpdate(sql);
   } catch (Exception e) {
     e.printStackTrace();
   }
 }
 @Override
 public void atualizar(Conta c) {
   try {
     Statement st;
     st = conexao.getConnection().createStatement();
     String sql =
         "update contas set saldo = " + c.getSaldo() + "where nro_contas = " + c.getNumero();
     st.executeUpdate(sql);
   } catch (Exception e) {
     e.printStackTrace();
   }
 }