private Persistencia() {
        base = BaseDatos.getInstancia();
        base.conectar("jdbc:mysql://localhost/dda", "root", "1234");
    }


 public void conectar(String url,String u,String p){
        try {
            conexion = DriverManager.getConnection(url, u, p);
            stmt = conexion.createStatement();
        } catch (SQLException ex) {
            System.out.println("Error al conectar:" + ex.getMessage());
        }
    }