AWT-EventQueue-1 on JList

Question

I finally got the code to compile correctly. However, there is a probably. I set the listbox to add a mouselistener and all but I get a huge error:

I added this, primarily, to the code: listbox.addMouseListener(new MousePopupListener());

and when I run it, it works. However, when I RIGHT-CLICK on the JList I get this:

Row: 0
Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
    at inv$MousePopupListener.checkPopup(inv.java:91)
    at inv$MousePopupListener.mouseReleased(inv.java:84)
    at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:273)
    at java.awt.Component.processMouseEvent(Component.java:6267)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6032)
    at java.awt.Container.processEvent(Container.java:2041)
    at java.awt.Component.dispatchEventImpl(Component.java:4630)
    at java.awt.Container.dispatchEventImpl(Container.java:2099)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)

    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
    at java.awt.Container.dispatchEventImpl(Container.java:2085)
    at java.awt.Component.dispatchEvent(Component.java:4460)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)

    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)

    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Here's my code:

import java.awt.*;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseAdapter;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.*;
import javax.swing.event.*;

public class inv extends JApplet implements MouseListener
{
public JList listbox;
public JPopupMenu popup;
public JMenuItem item;

public void init()
{
    ActionListener menuListener = new ActionListener()
    {
        public void actionPerformed(ActionEvent event)
        {
            String invAction = event.getActionCommand();
            System.out.println("Popup menu item [" + invAction + "] was pressed.");
        }
    };

    JPopupMenu popup = new JPopupMenu();

    popup.add(item = new JMenuItem("Use"));
    item.setHorizontalTextPosition(JMenuItem.RIGHT);
    item.addActionListener(menuListener);
    popup.add(item = new JMenuItem("Drop"));
    item.setHorizontalTextPosition(JMenuItem.RIGHT);
    item.addActionListener(menuListener);
    popup.add(item = new JMenuItem("Cancel"));
    item.setHorizontalTextPosition(JMenuItem.RIGHT);
    item.addActionListener(menuListener);



    String listData[] =
    {
        "Item 1","Item 2","Item 3","Item 4"
    };

    listbox = new JList( listData );
    listbox.addMouseListener( new MouseAdapter()
    {
        public void mousePressed(MouseEvent e)
        {
            if ( SwingUtilities.isRightMouseButton(e) )
            {
                System.out.println("Row: " + getRow(e.getPoint()));
                listbox.setSelectedIndex(getRow(e.getPoint()));
            }
        }
    }
    );

    listbox.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    add(listbox);
    listbox.setVisible(true);
    listbox.setFocusable(false);


    listbox.addMouseListener(new MousePopupListener());
}

class MousePopupListener extends MouseAdapter
{
    public void mousePressed(MouseEvent e)
    {
        checkPopup(e);
    }

    public void mouseClicked(MouseEvent e)
    {
        checkPopup(e);
    }

    public void mouseReleased(MouseEvent e)
    {
        checkPopup(e);
    }

    private void checkPopup(MouseEvent e)
    {
        if (e.isPopupTrigger())
        {
            popup.show(inv.this, e.getX(), e.getY());
        }
    }
}

private int getRow(Point point)
{
    return listbox.locationToIndex(point);
}

public void mouseEntered(MouseEvent e)
{
}

public void mouseReleased(MouseEvent e)
{
}

public void mousePressed(MouseEvent e)
{
}

public void mouseClicked(MouseEvent e)
{
}

public void mouseExited(MouseEvent e)
{
}

}

Answer 1

The issue is that you declare popup as a class variable and then create a local instance of popup in the init method. The result is that the class level popup is never set to anything and causes an NPE when you try to use it in checkPopup. Changing the line:

JPopupMenu popup = new JPopupMenu();

to

popup = new JPopupMenu();

should fix the issue.