Calculadora y Factorial en J2ME

package gui;
import javax.microedition.lcdui.ChoiceGroup;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.List;
import javax.microedition.lcdui.TextField;
import javax.microedition.midlet.*;

public class main extends MIDlet implements CommandListener{
 
      //Manejamos la pantalla con el display
      public Display display;
 
      //Aqui cargamos los texfield, list y group
      public Form screen;

      //Opciones del menu principal
      public Command salir, calcular;

      //Menu de opciones
      public List menu;

      public TextField dato1, operacion, dato2, total;
      public ChoiceGroup operador;
      public int mc;

      public main(){
         mc = 0;
         dato1 = new TextField("1er Numero: ", "", 15, TextField.NUMERIC);
         operador = new ChoiceGroup("Operacion:", ChoiceGroup.EXCLUSIVE);
         dato2 = new TextField("2do Numero", "", 15, TextField.NUMERIC);
         total = new TextField("Resultado", "", 15, TextField.UNEDITABLE);
         calcular = new Command("Calcular", Command.OK, 2);
         salir = new Command("Salir", Command.EXIT, 1);
     }      
 
     public void startApp() {
         display = Display.getDisplay(this);
         menu();
     }

     public void menu(){
         menu = new List("MENU PRINCIPAL", List.IMPLICIT);
         menu.append("Calculadora", null);
         menu.append("Factorial", null);
         menu.addCommand(salir);
         display.setCurrent(menu);
         menu.setCommandListener(this);
     }

     private void organizaCalculadora() {
         mc = 1;
         screen = new Form("Calculadora");
         display.setCurrent(screen);
         screen.append(dato1);
         operador.append("+", null);
         operador.append("-", null);
         operador.append("*", null);
         operador.append("/", null);
         screen.append(operador);
         screen.append(dato2);
         screen.append(total);
         screen.addCommand(calcular);
         screen.addCommand(salir);
         screen.setCommandListener(this);
     }

     private void organizaFactoria(){
         mc = 2;
         screen = new Form("Factorial");
         display.setCurrent(screen);
         screen.append(dato1);
         screen.append(total);
         screen.addCommand(calcular);
         screen.addCommand(salir);
         screen.setCommandListener(this);
     }

     public void pauseApp() {}

     public void destroyApp(boolean unconditional){notifyDestroyed();}

     public void commandAction(Command c, Displayable d) {

         //Si seleccionamos alguna de las opciones del menú
         if (d == menu){
             switch (menu.getSelectedIndex()){
                 case 0:
                     organizaCalculadora();
                     break;
                 case 1:
                     organizaFactoria();
                     break;
             }
         }

         //Si seleccionamos calculadora realizamos esta operación
         if (c == calcular && mc == 1){
             int n = 0;
             switch (operador.getSelectedIndex()){
                 case 0:
                     n = Integer.parseInt(dato1.getString()) + Integer.parseInt(dato2.getString());
                     break;
                 case 1:
                     n = Integer.parseInt(dato1.getString()) - Integer.parseInt(dato2.getString());
                     break;
                 case 2:
                     n = Integer.parseInt(dato1.getString()) * Integer.parseInt(dato2.getString());
                     break;
                 case 3:
                     n = Integer.parseInt(dato1.getString()) / Integer.parseInt(dato2.getString());
                     break;
             }
             total.setString(""+n);
         }
         
         //Si seleccionamos factorial realizamos esta operación
         if (c == calcular && mc == 2){
             int n,x = 0;
             n = Integer.parseInt(dato1.getString());
 
             if (n == 0 || n == 1){
                 n = 1;
             }else{
                 for (x = n; x >= 2; --x){
                    n = n * (x - 1) ;
                 }
             }
             total.setString(""+n);
         }

         if (c == salir){
             destroyApp(true);
         }
     }
}