I get a NullPointerException and don't know why

Hello folks!
As the subject, i get this error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

and i would like to know why.

This is my code


class Sfondo.java (a class that contains the background of the application)

Code:

---

public class Sfondo extends javax.swing.JFrame {
        public static JDesktopPane sfondo;
        private JLabel immagineAgenda;
        private JLabel immagineSfondo;

        public Sfondo() {
                super();
                setSfondo(sfondo);
        }
       
        public void setSfondo(JDesktopPane sfondo) {

                        setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
                        {
                                sfondo = new JDesktopPane();
                                getContentPane().add(sfondo, BorderLayout.CENTER);
                                {
                                        immagineAgenda = new JLabel();
                                        immagineSfondo = new JLabel();
                                        sfondo.add(immagineSfondo);
                                        sfondo.add(immagineAgenda);
                                        immagineSfondo.setIcon(new ImageIcon(getClass().getClassLoader().getResource("immagini/sig.gif")));
                                        immagineSfondo.setBounds(new Rectangle(510, 330, 400, 220));
                                        immagineAgenda.setIcon(new ImageIcon(getClass().getClassLoader().getResource("immagini/agenda.png")));
                                        immagineAgenda.setBounds(new Rectangle(0, 310, 315, 225));
                                        sfondo.setForeground(new java.awt.Color(0,128,255));
                                        sfondo.setBackground(new java.awt.Color(113,208,255));
                                        pack();
                                        this.setVisible(true);
                                        this.setSize(760, 560);
                                        this.setLocationRelativeTo(null);
                                }
                        }
               
                }
}

---

This is just the Avvio.java class, which simply contains the main method that calls Login class.

Code:

---


package avvio;
import javax.swing.SwingUtilities;

import avvio.Login;

public class Avvio {

        /**
        * @param args
        */
        public static void main(String[] args) {
                SwingUtilities.invokeLater(new Runnable() {
                        public void run() {
                                new Login();
                               
                        }
                });
        }
}

---

and, last, i have this Login.class that shows a simple label.

Code:

---


package avvio;

import javax.swing.JLabel;
import javax.swing.JDesktopPane;
import utilita.Sfondo;

public class Login {
        protected JLabel nome_utente;
               
        public Login() {
               
                graficaLogin(utilita.Sfondo.sfondo);
               
        }
       
               
        private void graficaLogin(JDesktopPane sfondo) {
                nome_utente = new JLabel();
                sfondo.add(nome_utente);
                nome_utente.setText("Nome utente");
                nome_utente.setBounds(166, 86, 164, 16);
                nome_utente.setFont(new java.awt.Font("Tahoma",0,20));
                nome_utente.setPreferredSize(new java.awt.Dimension(164, 16));
               
        }

}

---

I choosed (like a user on this forum told me to do) to pass the parameters so i would keep the background and i redraw it on every different part of my app. If anybody knows something lighter and easier, would be appreciated.
Anyway, i get a NullPointerException on line 19 of my Login.java class, which contains this

sfondo.add(nome_utente);

I think that the exception happens because sfondo not exists in memory. How can i solve this?
Thanks so much, i owe much to this forum... sorry for my bad english and for the dumb question and thanks again!

you have declared

Code:

---

public static JDesktopPane sfondo;

---

but you haven't defined it. I.E.

Code:

---

public static JDesktopPane sfondo = whatever;

---

Ok, solved with this:

Code:

---

public static JDesktopPane sfondo = new JDesktopPane();

---

and modifying my Login() function by adding a call to new Sfondo();

Thank you so muuuch! :)

Well, to tell you the truth, sfondo should not be static. Remove that keyword. and in your Login class simply do

Code:

---

Sfondo sfond = new SFondo();
graficaLogin(sfondo.sfondo);

---

Why it shouldn't be static? It won't be never modified by any class... if i got right the "static" concept. In addiction, if i make it non-static, i get the error "Cannot make a static reference to the non-static field Sfondo.sfondo"...

If you want it to be static, then don't initialise it in the constructor. Either initialise it with the declaration, or initialise it in a static block, but not in either the constructor or an instance message.

One rule of thumb to follow, if it doesn't need to be static, it shouldn't be.

Many thanks for the tip, i'll keep it like treasure.
But, i think it should be static: "sfondo" means "background" in english, so i think that the background will never be modified by any class, and it is called by every class, i don't need to construct it everytime... but (and this is a great probability) i kinda suck at Java, so i'm quite sure that i don't got right what static really means. BTW, i'm learning it day by day, so it's great to receive tips. ;)

If you want to learn more about static, click on the Suns' Java tutorial.

Here is the link.

Many thanks! Really appreciated. :cool:

If you have solve the problem please mark it as solved. It's really helpful to all other members.