Действия над дробями

package gui;
import drobi.Drob;
import java.util.*;
import javax.swing.*;
 
import java.awt.*;
import java.awt.event.*;
 
public class Gui {
public static void main(String []args){
new CalcFrame().setVisible(true);
 
}
}
class CalcFrame extends JFrame{ //наследование класса CalcFrame от JFrame
private Drob drob1;// = new fractionPack();
private Drob drob2;// = new fractionPack();
private DisplayPanel Dpanel;
 
public CalcFrame(){//вызывается при new
setTitle("Калькулятор");
setLayout(new BorderLayout());
setDefaultCloseOperation(EXIT_ON_CLOSE);
add(Dpanel = new DisplayPanel(), BorderLayout.NORTH);
add(new ButtonPanel(), BorderLayout.SOUTH);
dispose();
setResizable(false);
pack();
}//конструктор
 
class DisplayPanel extends JPanel{ //наследование класса DisplayPanel от JFrame
private JTextField display;
public DisplayPanel(){
display = new JTextField("", 20);
display.setEditable(false);
add(display);
setLayout(new FlowLayout());
}//конструктор
public JTextField getDisplay(){
return display;
}
public void setValue(String s){
display.setText(s);
}
}
class ButtonPanel extends JPanel{
public ButtonPanel(){
setLayout(new GridLayout(4, 4));
for(int i=0; i<10; i++)
addButton(""+i, new al1());
addButton("+", new al2());
addButton("-", new al2());
addButton("x", new al2());
addButton("/", new al1());
addButton("cmp", new al2());
addButton("enter", new enterAction());
addButton("div", new al2());
addButton("Cancel", new al2());
}
private void addButton(String lbl, ActionListener a){
JButton but = new JButton(lbl);
but.addActionListener(a);
this.add(but);
}
private class al1 implements ActionListener{
public void actionPerformed(ActionEvent event){
Dpanel.setValue(Dpanel.getDisplay().getText()+event.getActionCommand());
}
}
private void parsingDrobs(){
StringTokenizer st = new StringTokenizer(Dpanel.getDisplay().getText(),"/",false);
try{//потенциально опасное место
while(st.hasMoreTokens()){
/* drob1.setChislitel(Integer.parseInt(st.nextToken()));
drob1.setZnamenatel(Integer.parseInt(st.nextToken()));
drob2.setChislitel(Integer.parseInt(st.nextToken()));
drob2.setZnamenatel(Integer.parseInt(st.nextToken()));
*/
drob1 = new Drob(Integer.parseInt(st.nextToken()),Integer.parseInt(st.nextToken()));
drob2 = new Drob(Integer.parseInt(st.nextToken()),Integer.parseInt(st.nextToken()));
}
}catch(NullPointerException e){ //ловим исключение
JOptionPane.showMessageDialog(null,"На ноль делить нельзя", "ошибка" ,JOptionPane.WARNING_MESSAGE); }
 
}
 
private class enterAction implements ActionListener {
public void actionPerformed (ActionEvent event){
try{//потенциально опасное место
parsingDrobs();
}catch(NullPointerException e){ //ловим исключение
JOptionPane.showMessageDialog(null,"На ноль делить нельзя","ошибка",JOptionPane.WARNING_MESSAGE); // показываем диалоговое окно с ошибкой
}
}
}
 
private class al2 implements ActionListener{
public void actionPerformed(ActionEvent event){
if(event.getActionCommand().equals("Cancel")){
Dpanel.setValue("");
}else
if(event.getActionCommand().equals("+"))
Dpanel.setValue(drob1.sum(drob2).getChislitel()+"/"+drob1.sum(drob2).getZnamenatel());
else
if(event.getActionCommand().equals("-"))
Dpanel.setValue(drob1.razn(drob2).getChislitel()+"/"+drob1.razn(drob2).getZnamenatel());
else
if(event.getActionCommand().equals("x"))
Dpanel.setValue(drob1.umn(drob2).getChislitel()+"/"+drob1.umn(drob2).getZnamenatel());
else
if(event.getActionCommand().equals("div"))
Dpanel.setValue(drob1.del(drob2).getChislitel()+"/"+drob1.del(drob2).getZnamenatel());
else
if(event.getActionCommand().equals("cmp")){
if(drob1.sravn(drob2)==0)
Dpanel. setValue(drob1.getChislitel()+"/"+drob2.getZnamenatel()+"="+drob2.getChislitel()+"/"+drob2.getZnamenatel());
else 
if(drob1.sravn(drob2)==1)
Dpanel. setValue(drob1.getChislitel()+"/"+drob1.getZnamenatel()+">"+drob2.getChislitel()+"/"+drob2.getZnamenatel());
else
Dpanel. setValue(drob1.getChislitel()+"/"+drob2.getZnamenatel()+"<"+drob2.getChislitel()+"/"+drob2.getZnamenatel());
 
} 
}}}}