Unable to convert String to Double

I have a JTable where I load the data that I have in my BD, based on those records I make a simple operation, the data I enter and with which I do the operation I declared them as Double , at the moment to save the new data without performing the operation I get the following error, likewise if I do the operation does not register me and the same error:

  

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException:   java.lang.String can not be cast to java.lang.Double

private void registrarInventarioActionPerformed(java.awt.event.ActionEvent evt) {                                                    

    try {

        int rows = jTable1.getRowCount();
        System.out.println(rows);

            Class.forName("com.mysql.jdbc.Driver");


            String query = ("INSERT INTO ajuste_inventario_tmp(N_Ajuste,Categoria,ID,Referencia,Producto,UM,P_Compra,P_Venta,Teorico,Fisico,Diferencia,Nuevo_Inv) values(?,?,?,?,?,?,?,?,?,?,?,?)");
            PreparedStatement sentencia = conexion.prepareStatement(query);
        for(int row = 0; row<rows ; row++)
        {
            variables.N_Ajuste = (String) jTable1.getValueAt(row, 0);
            variables.Categoria = (String) jTable1.getValueAt(row, 1);
            variables.ID = (String) jTable1.getValueAt(row, 2);
            variables.Referencia = (String) jTable1.getValueAt(row, 3);
            variables.Producto = (String) jTable1.getValueAt(row, 4);
            variables.UM = (String) jTable1.getValueAt(row, 5);
            variables.P_Compra = (Double) jTable1.getValueAt(row, 6);
            variables.P_Venta = (Double) jTable1.getValueAt(row, 7);
            variables.Teorico = (Double) jTable1.getValueAt(row, 8);
            variables.Fisico = (Double) jTable1.getValueAt(row, 9);
            variables.Diferencia  = (Double) jTable1.getValueAt(row, 10);
            String Nuevo_Inv = (String) jTable1.getValueAt(row, 11);


            sentencia.setString(1, variables.N_Ajuste);           
            sentencia.setString(2, variables.Categoria);
            sentencia.setString(3, variables.ID);
            sentencia.setString(4, variables.Referencia);
            sentencia.setString(5, variables.Producto);
            sentencia.setString(6, variables.UM);
            sentencia.setDouble(7, variables.P_Compra);
            sentencia.setDouble(8, variables.P_Venta);
            sentencia.setDouble(9, variables.Teorico);
            sentencia.setDouble(10, variables.Fisico);
            sentencia.setDouble(11, variables.Diferencia);
            sentencia.setString(12, Nuevo_Inv);

            sentencia.executeUpdate();
        }
            JOptionPane.showMessageDialog(null, "Inventario registrado");
    } catch (HeadlessException | ClassNotFoundException | SQLException e) {
        JOptionPane.showMessageDialog(null, e);
        JOptionPane.showMessageDialog(null, "Error de conexión" + "\n" + e);
    }   
}    

It can be seen that I bring the variables of another class, these are already declared respectively.

Here I do the operation:

        double Fisico = Double.parseDouble(String.valueOf(tm.getValueAt(jTable1.getSelectedRow(),9)));
        double Teorico = Double.parseDouble(String.valueOf(tm.getValueAt(jTable1.getSelectedRow(),8)));

        if (Fisico == 0) {
            JOptionPane.showMessageDialog(null, "No hubo cambios en la columna Fisico.");
        }else{
            double Diferencia = (Teorico - Fisico);
            if (Diferencia < 0) {
                Diferencia = (Diferencia * (-1));
            }
            tm.setValueAt(Diferencia, jTable1.getSelectedRow(), 10);
            jTable1.clearSelection();

        }