JComboBox/JMenu problem

package hmc.it.model.table.filter.view;

import java.awt.BorderLayout;

import java.awt.event.ActionEvent;

import java.awt.event.ActionListener;

import javax.swing.JButton;

import javax.swing.JComboBox;

import javax.swing.JFrame;

import javax.swing.JPopupMenu;

/**

* This example demonstrates a NullPointer exception that occurs

* when a JComboBox is added as a Component to a JPopupMenu and

* then the combo box gets expanded to select another item.

*

* To see it in action, just compile and run this class. It is self-

* contained.

*

* Once it is started, click on the button to bring up the popup,

* then try to open the combo box. You'll get the following stack

* trace:

*

* java.lang.NullPointerException

* at javax.swing.plaf.basic.BasicPopupMenuUI$MouseGrabber.grabContainer(BasicPopupMenuUI.java:383)

* ....

*

* @author Vadim Yarovoy (vyarovoy@veripath.us)

* @date Jan, 10, 2007

*/

public class JComboBoxInJPopupMenuNullPointerExample {

/**

* @param args

*/

public static void main( String[] args ) {

//create frame

JFrame f = new JFrame();

f.setSize( 100, 100 );

f.setLocation( 500, 300 );

f.setTitle( "NullPointer example" );

//create button

JButton b = new JButton( "Click me" );

b.addActionListener( new ActionListener() {

public void actionPerformed( ActionEvent ev ) {

//create popup

JPopupMenu pm = new JPopupMenu();

//create the combo

JComboBox cb = new JComboBox(

new Object[] { "Open me", "Item 1", "Item 2", "Item 3" } );

//add cb to pm

pm.add( cb );

pm.show( (JButton)ev.getSource(), 10, 10 );

}

});

//add button to the frame

f.getContentPane().add( b, BorderLayout.CENTER );

f.show();

}

}

vyarovoya at 2007-7-8 2:22:41 >

Thanks for that, to tell you the truth I didn抰 really have any idea what the problem was. But now I see how two popup menus on top of each other could be a problem.

The reason I thought about using combo boxes was that I wanted he menu items to change dynamically.

I have found a way to use menu items and a HashMap to perform the same sort of behaviour.

Thanks again, and please feel free to play about with my little tester app.

import java.util.*;

import java.awt.event.*;

import javax.swing.*;

public class MenuBarTester extends JFrame

{

// The model to link the buttons on the favourites with there data object

private HashMap favMod = new HashMap();

// All the data objects to add at the start

private String[] data = new String[]{

"The","Cat","Sat","On","The","Mat"

};

// The menus

private JMenu menFav = new JMenu("Favourites");

private JMenuBar menuBar = new JMenuBar();

// Main

public static void main(String[] args)

{ new MenuBarTester(); }

public MenuBarTester()

{

// Set all the favourites in the fav menu

setFavs(data);

// Add

menuBar.add(menFav);

// Show frame

try

{

// Set look and feel to systems, might as well

UIManager.setLookAndFeel(

UIManager.getSystemLookAndFeelClassName());

} catch (Exception ex) {}

this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

this.setSize(100,100);

this.setJMenuBar(menuBar);

this.setVisible(true);

}

public void setFavs(Object[] inFavs)

{

// Remove all

favMod.clear();

for (int i=0;i<inFavs.length;i++)

{

System.out.println(i+" is "+inFavs[i]);

// Make a new button

JMenuItem favItem = new JMenuItem(inFavs[i].toString());

// This button now links to the TrackList in pos i

favMod.put(favItem,inFavs[i]);

// Add the action adapter

favItem.addActionListener(new FavItemActionAdapter());

// Add it to the menu

menFav.add(favItem);

}

}

private class FavItemActionAdapter implements ActionListener

{

public void actionPerformed(ActionEvent e)

{

// Get the source

Object bottom=e.getSource();

System.out.println("This button is linked to "+

favMod.get(bottom));

}

}

}

>

dfgstga at 2007-7-8 2:22:41 >