Java Tray Icon NPE

Question

I instanced a Java trayIcon and want to show a displayMessage on it with the following code:

trayIcon.displayMessage(_titel, _msg, TrayIcon.MessageType.INFO);

But instead of the Message, I get a NullpointerException and I don't now why?

This NPE:

Exception in thread "main" java.lang.NullPointerException
    at myPckg.Tray.showMsg(Tray.java:165)
    at myPckg.Main.main(Main.java:65)

In line 65 I instance the Class by Constructor, and in 155 I'm calling this object.

Thank you for helping!

Edit- The Code:

import java.awt.AWTException;
import java.awt.Image;
import java.awt.MediaTracker;
import java.awt.MenuItem;
import java.awt.PopupMenu;
import java.awt.SystemTray;
import java.awt.Toolkit;
import java.awt.TrayIcon;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.io.IOException;
import java.net.URL;

import javax.imageio.ImageIO;
import javax.swing.JOptionPane;

public class Tray {
//http://www.oracle.com/technetwork/articles/javase/index-136970.html   

public static MenuItem itmSpeicher = new MenuItem("0% von 0MB belegt");
public static MenuItem itmSyncStatus = new MenuItem("Synchronisation starten");
public static PopupMenu popup = new PopupMenu();
public static TrayIcon trayIcon = null;
public Tray(){
final TrayIcon trayIcon;


if (SystemTray.isSupported()) {

    SystemTray tray = SystemTray.getSystemTray();

    Image image = null;
    try {
        image = ImageIO.read(getClass().getResource("sync.gif"));
    } catch (IOException e2) {
        // TODO Auto-generated catch block
        e2.printStackTrace();
        new Thread(){ public void run(){ Main.syncProzess.reset();}}.start();
    }

    MouseListener mouseListener = new MouseListener() {

        public void mouseClicked(MouseEvent e) {
              Main.MainWindow.setVisible(true);
        }

        @Override
        public void mousePressed(MouseEvent e) {
            // TODO Auto-generated method stub

        }

        @Override
        public void mouseReleased(MouseEvent e) {
            // TODO Auto-generated method stub

        }

        @Override
        public void mouseEntered(MouseEvent e) {
            // TODO Auto-generated method stub

        }

        @Override
        public void mouseExited(MouseEvent e) {
            // TODO Auto-generated method stub

        }


    };

    ActionListener exitListener = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            int x = JOptionPane.showConfirmDialog(null, "Synchronisation wirklich beenden?");
            if(x==0){
                System.out.println("Beende... "+x);
                System.exit(0);
            }
        }
    };

    ActionListener itmSyncStatusListener = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            Main.syncProzess.starte();
        }
    };
    ActionListener itmFolderListener = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            try {
                Runtime.getRuntime().exec("explorer.exe "+ Main.syncDir );
            } catch (IOException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
                new Thread(){ public void run(){ Main.syncProzess.reset();}}.start();
            }

        }
    };
    ActionListener itmSettingListener = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            Main.MainWindow.setVisible(true);
        }
    };


    MenuItem defaultItem = new MenuItem("Programm beenden");
    defaultItem.addActionListener(exitListener);

    itmSyncStatus.addActionListener(itmSyncStatusListener);

    MenuItem itmFolder = new MenuItem(Main.programmName + " Ordner öffnen");
    itmFolder.addActionListener(itmFolderListener);

    MenuItem itmSettings = new MenuItem("Einstellungen");
    itmFolder.addActionListener(itmSettingListener);

    popup.add(itmFolder);
    popup.add(itmSpeicher);
    popup.add(itmSettings);
    popup.add(itmSyncStatus);
    popup.add(defaultItem);


    trayIcon = new TrayIcon(image, Main.programmName, popup);

    ActionListener doubleClick = new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            try {
                Runtime.getRuntime().exec("explorer.exe "+ Main.syncDir );
            } catch (IOException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
                new Thread(){ public void run(){ Main.syncProzess.reset();}}.start();
            }
        }
    };


    trayIcon.setImageAutoSize(true);
    trayIcon.addActionListener(doubleClick);
    trayIcon.addMouseListener(mouseListener);

    try {
        tray.add(trayIcon);
    } catch (AWTException e) {
        System.err.println("TrayIcon konnte nicht erstellt werden!");
        new Thread(){ public void run(){ Main.syncProzess.reset();}}.start();
    }
    showMsg("Hello", "It's working");

} else {
    //Not supportet
}


}

public void showMsg(String _titel, String _msg){
    trayIcon.displayMessage(_titel, _msg, TrayIcon.MessageType.INFO);
}

}

Answer 1

Your static trayIcon is null. You create in the constructor of Tray a new, second trayIcon, but your showMsg-Method uses the static trayIcon not the constructor local.

Remove in your Tray-constructor the first line:

final TrayIcon trayIcon;

and now you initialize the static reference.