GUI

import java.awt.*;import java.awt.event.*;import javax.swing.*;public class GUI extends JFrame implements ActionListener {        private JLabel jlbArtiest, jlbTitel, jlbLabel, jlbVoorraad;    private JTextField jtfArtiest, jtfTitel, jtfLabel, jtfVoorraad;    private JButton jbtAdd, jbtFind, jbtClear, jbtDelete, jbtUpdate, jbtList;        private JMenuBar menuBar;    private JMenu menuBestand, menuBewerken;    private JMenuItem itemNieuw, itemOpen, itemSluit, itemClear, info;    private JFrame frame;    private Winkel winkel = new Winkel();    /** Creates a new instance of GUI */    public GUI() {        this.jlbArtiest = new JLabel( "Artiest:");        this.jlbTitel = new JLabel( "Titel:");        this.jlbLabel = new JLabel( "Label:");        this.jlbVoorraad = new JLabel( "Voorraad:");            this.jtfArtiest = new JTextField( 30 );        this.jtfTitel = new JTextField( 30 );        this.jtfLabel = new JTextField( 20 );        this.jtfVoorraad = new JTextField( 5 );                this.jbtAdd = new JButton( "Add");        this.jbtDelete = new JButton( "Delete");        this.jbtFind = new JButton( "Find");        this.jbtClear = new JButton( "Clear");        this.jbtList = new JButton( "List");                this.menuBar = new JMenuBar();                menuBestand = new JMenu( "Bestand" );        menuBar.add( menuBestand );        info = new JMenuItem("info");        itemNieuw = new JMenuItem( "Nieuw" );        menuBestand.add( itemNieuw );        itemOpen = new JMenuItem( "Open" );        menuBestand.add( itemOpen );        itemSluit = new JMenuItem( "Sluit ");        menuBestand.add( itemSluit );        menuBestand.add(info);        menuBewerken = new JMenu( "Bewerken" );        menuBar.add( menuBewerken );                itemClear = new JMenuItem( "Clear" );        menuBewerken.add( itemClear );                setJMenuBar( menuBar );               setLayout( new FlowLayout() );                add( this.jlbArtiest );        add( this.jtfArtiest );        add( this.jlbTitel );        add( this.jtfTitel );        add( this.jlbLabel );        add( this.jtfLabel );        add( this.jlbVoorraad );        add( this.jtfVoorraad );        add( this.jbtAdd );        add( this.jbtFind );        add( this.jbtDelete );        add( this.jbtClear );        add( this.jbtList );                jbtAdd.addActionListener( this );        jbtFind.addActionListener( this );        jbtDelete.addActionListener( this );        jbtClear.addActionListener( this );        jbtList.addActionListener( this );        info.addActionListener(this);        itemNieuw.addActionListener( this );        itemOpen.addActionListener( this );        itemSluit.addActionListener( this );        itemClear.addActionListener( this );                this.setTitle( "CD Winkel Plato");        this.setSize( 410, 200 );        this.setVisible( true );    }        public void actionPerformed( ActionEvent e ) {        if( e.getSource() == this.jbtAdd ) performAdd();        if( e.getSource() == this.jbtClear ) performClear();        if( e.getSource() == this.jbtFind ) performFind();        if( e.getSource() == this.jbtDelete ) performDelete();        if( e.getSource() == this.jbtList ) performList();        if( e.getSource() == this.itemNieuw ) performNieuw();        if( e.getSource() == this.itemOpen ) performOpen();        if( e.getSource() == this.itemSluit ) performSluit();        if( e.getSource() == this.itemClear ) performClear();        if( e.getSource() == this.info) geefInfo();    }        private void geefInfo(){        frame = new JFrame();        Info informatie = new Info(frame);        informatie.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE);        informatie.setSize(300, 300);        informatie.setVisible(true);            }        private void performNieuw() {      winkel.nieuw();      this.performClear();    }            private void performOpen() {        winkel.read("cd.dat");    }            private void performSluit() {        winkel.write("cd.dat");    }            private void performAdd() {        Cd cd = winkel.findCd( this.jtfArtiest.getText(), this.jtfTitel.getText() );                if( cd != null )             JOptionPane.showMessageDialog( this,                     "CD bestaat al!",                     "Foutmelding", JOptionPane.ERROR_MESSAGE );        else {            int voorraad;            try            {              voorraad = Integer.parseInt(this.jtfVoorraad.getText());              winkel.addCd( this.jtfArtiest.getText(), this.jtfTitel.getText(), this.jtfLabel.getText(), voorraad );              jtfVoorraad.setBackground( Color.white );            } catch (NumberFormatException ex) {              jtfVoorraad.setBackground( Color.red );              JOptionPane.showMessageDialog( this,                       "Waarde veld voorraad niet valide!",                       "Foutmelding", JOptionPane.ERROR_MESSAGE );            }        }    }            private void performClear() {        this.jtfArtiest.setText( "" );        this.jtfTitel.setText( "" );        this.jtfLabel.setText( "" );        this.jtfVoorraad.setText( "" );    }            private void performFind() {        Cd cd = winkel.findCd( this.jtfArtiest.getText(), this.jtfTitel.getText() );                if( cd == null ) {            JOptionPane.showMessageDialog( this,                     "CD niet gevonden!",                     "Foutmelding", JOptionPane.ERROR_MESSAGE );            performClear();        }        else {            jtfArtiest.setText( cd.getArtiest() );            jtfTitel.setText( cd.getTitel() );            jtfLabel.setText( cd.getLabel() );            jtfVoorraad.setText( "" + cd.getVoorraad() );        }    }            private void performDelete() {        Cd cd = winkel.findCd( this.jtfArtiest.getText(), this.jtfTitel.getText() );                if( cd == null ) {            JOptionPane.showMessageDialog( this,                     "CD niet gevonden!",                     "Foutmelding", JOptionPane.ERROR_MESSAGE );        }         else {          int option = JOptionPane.showConfirmDialog( this,                    "Deze CD echt verwijderen?",                    "Vraag",                    JOptionPane.YES_NO_OPTION );          if( option == JOptionPane.YES_OPTION ) {            winkel.deleteCd( this.jtfArtiest.getText(), this.jtfTitel.getText() );            performClear();          }        }    }            private void performList() {      ListDialog ld = new ListDialog( this, winkel.getAll() );    }        /**     * @param args the command line arguments     */    public static void main( String[] args ) {        GUI gui = new GUI();        gui.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );    }    }