I get a NullPointerException and don't know why

[hendrix79]

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)

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.

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.

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!

[masijade]

you have declared

public static JDesktopPane sfondo;

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

public static JDesktopPane sfondo = whatever;

[hendrix79]

Ok, solved with this:

public static JDesktopPane sfondo = new JDesktopPane();

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

Thank you so muuuch! :)

[masijade]

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

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

[hendrix79]

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"...

[masijade]

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.

[hendrix79]

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. ;)

[Eranga]

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

Here is the link.

[hendrix79]

Many thanks! Really appreciated. :cool:

[Eranga]

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