Question

Hello I have created a application with several buttons in it. but when I press a button I get the NullPointerException. The strange thing here is that nothing is empty ( null )

here a code example

public class MuseumPanel extends JPanel implements ActionListener {

private JTextField kaartnummer;
private JTextField uur, minuut;
private JButton aankomst, vertrek, overzicht, sluiting;

private int hour;
private int minute;
private final int REFRESH = 1000;

MuseumRegistratie museum;

public MuseumPanel(MuseumRegistratie museum) {
    // zorg ervoor dat de huidige tijd wordt opgehaald.
    javax.swing.Timer timer = new javax.swing.Timer(REFRESH, this);
    timer.start();

    kaartnummer = new JTextField(15);
    uur = new JTextField(2);
    minuut = new JTextField(2);
    aankomst = new JButton("Komt binnen");
    aankomst.addActionListener(this);
    vertrek = new JButton("Vertrekt");
    vertrek.addActionListener(this);
    overzicht = new JButton("aantal aanwezig");
    overzicht.addActionListener(this);
    sluiting = new JButton("sluiting");
    sluiting.addActionListener(this);

    add(new JLabel("Kaartnummer"));
    add(kaartnummer);
    add(new JLabel("tijdstip van aankomst of vertrek "));
    add(uur);
    add(new JLabel(" uur en "));
    add(minuut);
    add(new JLabel(" minuten"));
    add(aankomst);
    add(vertrek);
    add(overzicht);
    add(sluiting);
}

@Override
public void actionPerformed(ActionEvent e) {
    Calendar now = Calendar.getInstance();
    hour = now.get(Calendar.HOUR_OF_DAY);
    minute = now.get(Calendar.MINUTE);

    uur.setText("" + hour);
    minuut.setText("" + minute);

    // aankomst
    if(e.getSource() == aankomst) {
        try {

            museum.checkIn(kaartnummer.getText(), hour, minute);
        } catch (NullPointerException ex) {

            System.out.println("cardnumber: " + kaartnummer.getText() + " hour " + hour + " minute " + minute);
        }
    }
    // vertrek
    if(e.getSource() == vertrek) {
        museum.checkOut(kaartnummer.getText(), hour, minute);
    }
    // overzicht
    if(e.getSource() == overzicht) {
        museum.getAantalAanwezig();
    }
    // sluiting
    if(e.getSource() == sluiting) {
        museum.sluitRegistratie();
    }
}
}

When pressing this button for example I get the exception with every variable correctly.. Does anyone know how this appears and how to solve it?

Answer 1

You don’t seem to be assigning a value to museum in your constructor, yet you dereference it in a lot of places. You’d want to do this:

this.museum = museum;

somewhere in your constructor. Alternatively, rename the variable so you don’t accidentally do museum = museum, which would have no effect.