[Hilfe] Euro Umrechner für Schule

package umrechner;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

/**
 * <p>Überschrift: Umrechner von Euro in Dollar</p>
 *
 * <p>Beschreibung: kleines Fensterproggi, was Euro in Doller und umgedreht rechnet</p>
 *
 * <p>Copyright: Copyright (c) 2006</p>
 *
 * <p>Organisation: </p>
 *
 * @author HARIBO
 * @version 1.0
 */
public class Euro extends JFrame
{

 JTextField euro=new JTextField();
 JTextField dollar=new JTextField();
 JButton button1=new JButton("Euro in Dollar");
 JButton button2=new JButton("Dollar in Euro");
 JButton button3=new JButton("und tschüss");
 public Euro(String titel)
 {
 super(titel);
 setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
 Container cont1= getContentPane();
 JPanel panel=new JPanel();
 panel.setLayout(new GridLayout(3,3));
 cont1.add(panel);


 JPanel panel1=new JPanel();
 button1.setPreferredSize(new Dimension(150,30));
 panel1.add(button1);
 panel.add(panel1);


 JPanel panel4=new JPanel();
 JLabel label4=new JLabel("Eingabe in €");
 euro.setPreferredSize(new Dimension(100,20));
 panel4.add(label4);
 panel4.add(euro);
 panel.add(panel4);

 JPanel panel2=new JPanel();
 button2.setPreferredSize(new Dimension(150,30));
 panel2.add(button2);
 panel.add(panel2);

 JPanel panel5=new JPanel();
 JLabel label3=new JLabel("Eingabe in $");
 dollar.setPreferredSize(new Dimension(100,20));
 panel4.add(label3);
 panel4.add(dollar);
 panel.add(panel5);

 JPanel panel3=new JPanel();
 button3.setPreferredSize(new Dimension(150,30));
 panel3.add(button3);
 panel.add(panel3);


 ActionListener listener=new ActionListener()
 {
 public void actionPerformed(ActionEvent e)
 {
 euro_in_dollar();
 }
 };
 button1.addActionListener(listener);


 ActionListener listener1=new ActionListener()
 {
 public void actionPerformed(ActionEvent e)
 {
 dollar_in_euro();
 }
 };
 button2.addActionListener(listener1);


 ActionListener listener2=new ActionListener()
 {
 public void actionPerformed(ActionEvent e)
 {
 System.exit(0);
 }
 };
 button3.addActionListener(listener2);
 }

 public void euro_in_dollar()
 {
 try
 {
 //1 Euro=1,2713 USD
 double eingabe_euro=Double.parseDouble(euro.getText());
 double erg=eingabe_euro/(1.2713);
 JOptionPane.showMessageDialog(null,"Ergebnis lautet: "+erg);
 }
 catch (NumberFormatException e)
 {
 JOptionPane.showMessageDialog(null,"Du hast leider keine Zahl eingeben!!!:D");
 }

 }

 public void dollar_in_euro()
 {
 try
 {
 double eingabe_dollar=Double.parseDouble(dollar.getText());
 double erg=1.2713*eingabe_dollar;
 JOptionPane.showMessageDialog(null,"Ergebnis lautet: "+erg);
 }
 catch (NumberFormatException e)
 {
 JOptionPane.showMessageDialog(null,"Du hast leider keine Zahl eingeben!!!:D");
 }
 }

 public void centerFrame()
 {
 Dimension d=Toolkit.getDefaultToolkit().getScreenSize();
 int mx=d.width;
 int my=d.height;
 int fx=getSize().width;
 int fy=getSize().height;
 setLocation((mx-fx)/2,(my-fy)/2);
 }
 public static void main(String[] args)
 {
 try
 {
 UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName()); //Windows Look and feel
 }
 catch (Exception e)
 {
 e.printStackTrace();
 }

 Euro frame = new Euro("Umrechner € <> $");
 frame.setSize(400,200);
 frame.setLocation(450,450);
 frame.centerFrame();
 frame.setVisible(true);
 frame.setResizable(false);
 }
}