Develop Reference

invisible JMenuBar, Accelerator not working

My Program has a JMenuBar with JMenuItems.
They have a ActionListener, and I set a Shortcut with setAccelerator.
Now I am hiding the menu bar when the window become unfocused, to get more space for a displayed image.
But after the first hiding of the menubar, the hotkeys just stop working.
How can I fix that?
I created a little example code to illustrate that strange behavior:
import javax.swing.*;
import java.awt.event.*;
class Example extends JFrame{
public static void main(String[] args) {
new Example(); //main is static
}
static JMenuBar menubar; //be accessable for the ActionListener
Example() {
//JPanel
this.setSize(50,50);
this.setVisible(true);
//Menubar, static
menubar = new JMenuBar();
this.setJMenuBar(menubar);
//Menu
JMenu filemenu = new JMenu("File");
menubar.add(filemenu);
//Item
JMenuItem menuitem = new JMenuItem("Do Something...");
filemenu.add(menuitem);
menuitem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_D, KeyEvent.SHIFT_DOWN_MASK)); // Shift + D
menuitem.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
System.out.println("Action!");
}
});
JButton button = new JButton("Show/Hide menubar");
this.add(button);
button.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
System.out.println("Toggle Bar!");
menubar.setVisible(!menubar.isVisible()); //Toggle
}
});
}
}
For reference:
I'm using Java 1.7.0_60-ea (Java 7) on a Mac.
But this error occurs independent of using the Mac native menu bar or the normal java menu bar inside the JFrame.

You could try to add global keybindings. How to add keybindings is explained here.
Here is an example of what you could do:
//Any component that is always visible in the window (like the image)
JComponent c;
//Get input and action map
InputMap imap = c.getInputMap(WHEN_IN_FOCUSED_WINDOW);
ActionMap amap = c.getActionMap();
//Put keybinding and action
imap.put(KeyStroke.getKeyStroke("shift D"), "doSomething");
amap.put("doSomething", anAction);
Note that it only works in the focused window. But should work regardless of the menubar being visible or not.

setMnemonic() and call a method by pressing the key

I have to run a method manually when pressing the key Alt+H
if("The key pressed==(Alt+H)"){
callMethod();
}
public void callMethod(){
//Some codes here
}
How I can actually do this in Java. Please give me a simple way to do this.

It's worth reading here about Oracle Tutorial - Enabling Keyboard Operation where it is explained in details along with sample.
Read more about on Oracle Tutorial - How to Use Key Bindings
Some example directly from the above tutorial:
//Setting the mnemonic when constructing a menu item:
menuItem = new JMenuItem("A text-only menu item",
KeyEvent.VK_H);
//Setting the mnemonic after creation time:
menuItem.setMnemonic(KeyEvent.VK_H);
//Setting the accelerator:
menuItem.setAccelerator(KeyStroke.getKeyStroke(
KeyEvent.VK_H, ActionEvent.ALT_MASK));
Read more here Oracle Tutorial - How to Use Buttons, Check Boxes, and Radio Buttons
Sample code: (Alt-H would click the Middle button)
JButton b2 = new JButton("Middle button", middleButtonIcon);
b2.setMnemonic(KeyEvent.VK_H);

If using menus then you can use setMnemonic(), see How to Use Menus for examples. Another option is to use Key Bindings. For example:
import java.awt.event.*;
import javax.swing.*;
public class TestKeys {
private static void createAndShowGUI() {
final JFrame frame = new JFrame("Keys");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
KeyStroke escapeKeyStroke = KeyStroke.getKeyStroke(KeyEvent.VK_H, InputEvent.ALT_MASK);
Action testAction = new AbstractAction() {
public void actionPerformed(ActionEvent e) {
JOptionPane.showMessageDialog(frame, "Alt-H pressed");
}
};
frame.getRootPane().getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(escapeKeyStroke, "TestAction");
frame.getRootPane().getActionMap().put("TestAction", testAction);
JLabel label = new JLabel("Hit Alt-H");
label.setBorder(BorderFactory.createEmptyBorder(20, 20, 20, 20));
frame.add(label);
frame.setLocationByPlatform(true);
frame.pack();
frame.setVisible(true);
}
public static void main(String args[]) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGUI();
}
});
}
}

Best method is to use setMnemonic() because its the the simplest.
check this article for more http://www.herongyang.com/Swing/JMenuBar-Set-Keyboard-Mnemonics-on-Menu-Items.html
private JMenu getColorMenu() {
JMenu myMenu = new JMenu("Color");
ButtonGroup myGroup = new ButtonGroup();
JRadioButtonMenuItem myItem = new JRadioButtonMenuItem("Red");
myItem.setSelected(true);
myItem.setMnemonic(KeyEvent.VK_R);
myItem.addActionListener(this);
myItem.addMenuKeyListener(this);
myGroup.add(myItem);
myMenu.add(myItem);
myItem = new JRadioButtonMenuItem("Green");
myItem.setMnemonic(KeyEvent.VK_G);
myItem.addActionListener(this);
myItem.addMenuKeyListener(this);
myGroup.add(myItem);
myMenu.add(myItem);
myItem = new JRadioButtonMenuItem("Blue");
myItem.setMnemonic(KeyEvent.VK_B);
myItem.addActionListener(this);
myItem.addMenuKeyListener(this);
myGroup.add(myItem);
myMenu.add(myItem);
return myMenu;

How do you create an ActionListener for multiple JMenuItems?

I'm having difficulty using anonymous inner classes with actionListener. Can someone explain to me what is wrong with my code and how to use anonymous inner classes with actionListener. I'm trying to make a menu bar in one class and the action listener in the other. I ran into some difficulty when I tried using anonymous inner classes. The java website wasn't clear. Can you please explain it to me and help me fix my code.
public class Listener implements ActionListener {
HangmanView hangmanView = new HangmanView();
JFrame dialogFrame = new JFrame();
ImageIcon logo = new ImageIcon("logo.png");
public void listener1() {
hangmanView.getMenuItem().addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {// right click key
JOptionPane.showMessageDialog(dialogFrame, "Developer: Joe"
, "Developer",
JOptionPane.INFORMATION_MESSAGE, logo);
}// end actionPerformed method
});
}
}
another class:
public class HangmanView {
public JMenuItem getMenuItem() {
JMenuItem menuItem = new JMenuItem("Developer", KeyEvent.VK_T);
menuItem.addActionListener(new Listener());
return menuItem;
}
public JMenuBar menuBar() {
JMenuBar menuBar = new JMenuBar();
JMenu menu = new JMenu("File");
menuBar.add(menu);
menu.add(getMenuItem());// return here
return menuBar;
}

If you're trying to implement the listener for different JMenuItems, what I would do instead is create a custom Action class that you can use for multiple JMenuItems, as JMenuItems are a good example of when to use an Action.
private class MyAction extends AbstractAction {
String name;
public MyAction(String name, Icon icon) {
super(name, icon);
this.name = name;
}
public MyAction(String name, Icon icon, String desc,
Integer mnemonic, KeyStroke accelorator) {
super(name, icon);
putValue(Action.SHORT_DESCRIPTION, desc);
putValue(Action.MNEMONIC_KEY, mnemonic);
putValue(Action.ACCELERATOR_KEY, accelorator);
this.name = name;
}
#Override
public void actionPerformed(ActionEvent e) {
switch (name) {
case "menu1Action":
// do something for menuItem1
break;
case "menu2Action":
// do something for menuItem2
break;
case "menu3Action":
// do something for menuItem3
break;
}
}
}
Have this class as an inner class of HangmanView. You can then create an instance of this custom Action class for each JMenuItem. Here's a example
Action menu1Action = new MyAction(
/* arg 1 */ "menu1Action",
/* arg 2 */ someIcon,
/* arg 3 */ "Some Short description of the action",
/* arg 4 */ new Integer(KeyEvent.VK_T),
/* arg 5 */ KeyStroke.getKeyStroke(KeyEvent.VK_T, ActionEvent.CTRL_MASK));
The first argument is the name of the action. This name will be the name that you will see in the menu
The second argument is the icon that you will see in the menu next to the name.
The third argument is the Description of the menu item action
The fourth argument is the Mnemonic (i.e. Alt + T).
The fifth argument is the Accelerator (i.e. Ctrl + T).
When you add an Action to a JMenu, the title of that Action will automatically get placed as what you see in the JMenu. So all you need to do is add this custom Action to your JMenu. You don't ever have to actually create a JMenuItem at all. The Action will serve as the replacement for the JMenuItem. Just add all your MyAction objects to the JMenu.
menu.add(menu1Action);
What I have left out, is the implementation for each separate switch case in the actionPerformed. The case will be what you name the action in the constructor. Because I named the Action "menu1Action", I should have the corresponding name in the the switch case. In that case, you can do your JOptionPane or what ever else you wish to perform when that JMenuItem is clicked or accessed by keyboard.
Another great benefit of using an Action is that it can serve multiple purposes. With the same MyAction menu1Action you created, you can use the same Action for a JToolBar. Without any alteration to the above menu1Action, you could just do this:
JTooBar toolbar = new JToolBar();
toolbar.add(menu1Action);
Now in your tool bar and in your menu item, you have the same action. The tool bar will only show the icon and not the name.
Here is an example. What I do is create three different MyAction objects. One for left-alignment, one for center-alignment, and one for right-alignment. Each of these actions is used three separate times for three separate components, a menu item, a toll bar and a button
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class ActionInterfaceDemo extends JFrame {
static JPanel buttonPanel = new JPanel();
static FlowLayout flowLayout = new FlowLayout();
public ActionInterfaceDemo(){
ImageIcon centerIcon = new ImageIcon(
ActionInterfaceDemo.class.getResource("image/centeralignment.png"));
ImageIcon rightIcon = new ImageIcon(
ActionInterfaceDemo.class.getResource("image/rightalignment.png"));
ImageIcon leftIcon = new ImageIcon(
ActionInterfaceDemo.class.getResource("image/leftalignment.png"));
Action leftAction = new MyAction("Left", leftIcon,
"Left alignment for the buttons in the panel",
new Integer(KeyEvent.VK_L),
KeyStroke.getKeyStroke(KeyEvent.VK_L, ActionEvent.CTRL_MASK));
Action rightAction = new MyAction("Right", rightIcon,
"Right alignment for the buttons in the panel",
new Integer(KeyEvent.VK_R),
KeyStroke.getKeyStroke(KeyEvent.VK_R, ActionEvent.CTRL_MASK));
Action centerAction = new MyAction("Center", centerIcon,
"Center alignment for the buttons in the panel",
new Integer(KeyEvent.VK_C),
KeyStroke.getKeyStroke(KeyEvent.VK_C, ActionEvent.CTRL_MASK));
JMenuBar menuBar = new JMenuBar();
JMenu menuAlignment = new JMenu("Alignment");
setJMenuBar(menuBar);
menuBar.add(menuAlignment);
menuAlignment.add(leftAction);
menuAlignment.add(centerAction);
menuAlignment.add(rightAction);
JToolBar toolBar = new JToolBar("Alignment");
toolBar.setBorder(BorderFactory.createLineBorder(Color.BLUE));
toolBar.add(leftAction);
toolBar.add(centerAction);
toolBar.add(rightAction);
buttonPanel.setLayout(flowLayout);
JButton jbtLeft = new JButton(leftAction);
JButton jbtCenter = new JButton(centerAction);
JButton jbtRight = new JButton(rightAction);
buttonPanel.add(jbtLeft);
buttonPanel.add(jbtCenter);
buttonPanel.add(jbtRight);
add(toolBar, BorderLayout.EAST);
add(buttonPanel, BorderLayout.CENTER);
pack();
setLocationRelativeTo(null);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setVisible(true);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable(){
public void run(){
new ActionInterfaceDemo();
}
});
}
private class MyAction extends AbstractAction {
String name;
public MyAction(String name, Icon icon, String desc,
Integer mnemonic, KeyStroke accelorator) {
super(name, icon);
putValue(Action.SHORT_DESCRIPTION, desc);
putValue(Action.MNEMONIC_KEY, mnemonic);
putValue(Action.ACCELERATOR_KEY, accelorator);
this.name = name;
}
#Override
public void actionPerformed(ActionEvent e) {
switch (name) {
case "Left":
flowLayout.setAlignment(FlowLayout.LEFT);
break;
case "Right":
flowLayout.setAlignment(FlowLayout.RIGHT);
break;
case "Center":
flowLayout.setAlignment(FlowLayout.CENTER);
break;
}
buttonPanel.revalidate();
}
}
}
You can press the "Left" in either the menu, the toolbar, or the button and they will produce the same result, as they are derived from the same Action.
Here are the images I used if you want to test it out
Note You don't have to use either of these exact constructors. You can create your own with different arguments. This is just a custom one I like to use.
Aslo See How to use Action tutorial

Listener, being an ActionListener through its inheritance hierarchy,
public class Listener implements ActionListener {
needs to implement an actionPerfomed(ActionEvent) method
#Override
public void actionPerformed(ActionEvent e) {
// implement it
}
However, since you seem to be adding an anonymous ActionListener, just don't have your Listener class implement ActionListener. Remove that bit.

Your object-oriented programming is all over the place. Your code in the OP looks like it's supposed to be some kind of GUI with different classes somehow working together but it's just creating new objects all over the place. There are too many reasons it doesn't work. I'd suggest you stick to something simple until you have a better grasp on how all this works.
You also got a very good suggestion in your other question that's a lot like this one but you haven't really followed it. Your code does something completely different.
Here is a very basic GUI. You have one object that has everything as fields. Everything is in one place. The containing object is the listener and decides what to do based on the event source. You should stick with a design like this until you are more comfortable with OOP.
public class HangmanView
implements ActionListener {
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
new HangmanView().setFrameVisible(true);
}
});
}
private JFrame theFrame = new JFrame("Main Window");
private JPanel theContent = new JPanel();
private JMenuBar theBar = new JMenuBar();
private JMenu fileMenu = new JMenu("File");
private JMenuItem exitMenuItem = new JMenuItem("Exit");
private JMenuItem devMenuItem = new JMenuItem("Developer");
public HangmanView() {
assert SwingUtilities.isEventDispatchThread();
exitMenuItem.addActionListener(this);
devMenuItem.addActionListener(this);
fileMenu.add(exitMenuItem);
theBar.add(fileMenu);
theBar.add(devMenuItem);
theContent.setPreferredSize(new Dimension(500, 500));
theFrame.setJMenuBar(theBar);
theFrame.setContentPane(theContent);
theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
theFrame.pack();
theFrame.setLocationRelativeTo(null);
}
public void setFrameVisible(boolean vis) {
theFrame.setVisible(vis);
}
#Override
public void actionPerformed(ActionEvent ae) {
if(ae.getSource() == devMenuItem) {
showDevDiag();
} else if(ae.getSource() == exitMenuItem) {
systemExit();
}
}
private void showDevDiag() {
JOptionPane.showMessageDialog(
theFrame,
"Developer: Joe",
"Developer",
JOptionPane.INFORMATION_MESSAGE,
null
);
}
private void systemExit() {
System.exit(0);
}
}

if I should use anonymous class, that's what should be done:
public static void main(String args[]){
/*bla bla bla...*/
JButton button1=new JButton("button1");
button1.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent ae){
/*bla bla bla...*/
}
};
}
On the other hand, I always do this:
public class Class implements Runnable,ActionListener
{
private static Map<Thread,ActionEvent> THREAD_ATTRIB
=new HashMap<Thread,ActionEvent> (0);
JButton button1=new JButton("button1");
JButton button2=new JButton("button2");
//constructor
public Class(){
this.button1.addActionListener(this);
this.button2.addActionListener(this);
}
#Override
public void actionPerformed(ActionEvent ae){
Thread thread=new Thread(this);
THREAD_ATTRIB.put(thread,ae);
thread.start();
}
#Override
public void run(){
ActionEvent ae=THREAD_ATTRIB.get(Thread.currentThread());
if(ae!=null){
Object source=ae.getSource();
if(source.equals(this.button1){
/*bla bla bla...*/
} else if(source.equals(this.button2){
/*bla bla bla...*/
}
} else{
/*bla bla bla...*/
}
}
}

JMenuItem shortcuts Ctrl-C / Ctrl-V or Ctrl-Insert / Shift-Insert don't work anymore

I have an older stand alone java swing-based app that uses JFrame with JMenuBar containing multiple Jmenu elements (with respective JMenuItem items).
After upgrading to the latest 1.6.0_41 (or 1.7.x) JVM on Windows (7 and vista) I've noticed that the menu item with the shortcut Ctrl-C (or Ctrl-Insert) doesn't receive its ActionEvent anymore if JTable is added to the frame. The menu ActionListener is invoked if the menu is accessed by the mouse click however. The shortcut works if JTable is removed. If I change the shortcut combination to something other than Ctrl-C or Ctrl-Insert (i.e. Ctrl-L) the ActionListener is invoked.
The way it used to work (I've just confirmed it with jvm 1.4, on Windows Vista - I know it's been awhile since that environment got any serious attention :) is that Ctrl-C will perform the standard copy to clipboard function inside of the JTable if the focus was inside of an editable field. Otherwise my menu ActionListener was invoked via shortcut assigned through setAccelerator() method.
It looks like JTable implementation changed in 1.6.* to process Ctrl-C bound event differently on Windows.
Running this app on Mac OS (JVM 1.6.0_43) I can see ActionListener is invoked via Ctrl-C shortcut. Although it might be because the JTable uses Command-C instead of Ctrl-C to copy to the clipboard under Mac OS.
I've extracted the relevant portion of the code that demonstrates the problem. Any suggestions are greatly appreciated.
public class TestFrame extends JFrame {
public TestFrame(String title) {
super(title);
}
private void init() {
getContentPane().setLayout(new BorderLayout());
addMenu();
addTable();
// Change default exit operation
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true);
}
private void addTable() {
JTable jTable = new JTable(createTableModel());
// Place table in JScrollPane
JScrollPane scrollPane = new JScrollPane(jTable);
// Add Table
add(scrollPane, BorderLayout.CENTER);
}
private TableModel createTableModel() {
Object[][] data = new Object[][]{
{new Date(), "First Row, 2nd column", "First Row, 3rd column"},
{new Date(), "Second Row, 2nd column", "Second Row, 3rd column"},
};
Object[] columnNames = new Object[]{"Date", "Type", "Description"};
DefaultTableModel model = new DefaultTableModel(data, columnNames) {
public boolean isCellEditable(int row, int column) {
return column != 0;
}
};
return model;
}
private void addMenu() {
// Create the menu bar.
JMenuBar menuBar = new JMenuBar();
setJMenuBar(menuBar);
JMenu editMenu = new JMenu("Edit");
menuBar.add(editMenu);
TestActionListener listener = new TestActionListener();
JMenuItem menuItem = null;
menuItem = new JMenuItem("Copy 1");
menuItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_INSERT, ActionEvent.CTRL_MASK));
menuItem.addActionListener(listener);
editMenu.add(menuItem);
menuItem = new JMenuItem("Copy 2");
menuItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_C, ActionEvent.CTRL_MASK));
menuItem.addActionListener(listener);
editMenu.add(menuItem);
menuItem = new JMenuItem("Copy 3");
menuItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_L, ActionEvent.CTRL_MASK));
menuItem.addActionListener(listener);
editMenu.add(menuItem);
}
public static void main(String[] args) {
TestFrame frame = new TestFrame("Test");
frame.init();
}
private static class TestActionListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
System.out.println("TestFrame.TestActionListener.actionPerformed(): e="+ e);
}
}
}

The problem is that your frame is not focused and there are no elements in your whole component hierarchy which has the focus, meaning that no one will "grab" the event and try to do something with it. Since JMenuItem's bind their shortcut to the input map JComponent.WHEN_IN_FOCUSED_WINDOW, your shortcut never "answers" the event.
To fix this, either put the focus on one of the component or directly on the JFrame (for example with frame.requestFocusInWindow();). Small example here:
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.KeyStroke;
import javax.swing.SwingUtilities;
public class TestFrame extends JFrame {
public TestFrame(String title) {
super(title);
}
private void init() {
getContentPane().setLayout(new BorderLayout());
addMenu();
addTable();
// Change default exit operation
setDefaultCloseOperation(EXIT_ON_CLOSE);
pack();
setVisible(true);
}
private void addTable() {
JTable jTable = new JTable();
// Place table in JScrollPane
JScrollPane scrollPane = new JScrollPane(jTable);
// Add Table
add(scrollPane, BorderLayout.CENTER);
}
private void addMenu() {
// Create the menu bar.
JMenuBar menuBar = new JMenuBar();
setJMenuBar(menuBar);
JMenu editMenu = new JMenu("Edit");
menuBar.add(editMenu);
TestActionListener listener = new TestActionListener();
JMenuItem menuItem = null;
menuItem = new JMenuItem("Copy 1");
menuItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_INSERT, KeyEvent.CTRL_MASK));
menuItem.addActionListener(listener);
editMenu.add(menuItem);
menuItem = new JMenuItem("Copy 2");
menuItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_C, KeyEvent.CTRL_MASK));
menuItem.addActionListener(listener);
editMenu.add(menuItem);
menuItem = new JMenuItem("Copy 3");
menuItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_L, KeyEvent.CTRL_MASK));
menuItem.addActionListener(listener);
editMenu.add(menuItem);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
TestFrame frame = new TestFrame("Test");
frame.init();
frame.requestFocusInWindow();
}
});
}
private static class TestActionListener implements ActionListener {
#Override
public void actionPerformed(ActionEvent e) {
System.out.println("TestFrame.TestActionListener.actionPerformed(): e=" + e);
}
}
}
Additional remarks:
Don't extend JFrame if not needed
Start your UI from the Event Dispatching Thread (EDT) by using SwingUtilities.invokeLater()

If you question is how to remove the Control+C binding from the table then you can do:
KeyStroke copy = KeyStroke.getKeyStroke("control C");
InputMap im = table.getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
im.getParent().remove(copy);
However, this will remove the binding from all tables.

How to create Combobox with multiselection?

I need to create a combo box with multi-selection, how to achieve that?

I know, that the question is rather old, but for those, who still looks for solution of this problem, try the following code:
public class ComboSelections {
public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, UnsupportedLookAndFeelException {
UIManager.setLookAndFeel((LookAndFeel) Class.forName("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel").newInstance());
final JPopupMenu menu = new JPopupMenu();
JMenuItem one = new JCheckBoxMenuItem("One");
JMenuItem two = new JCheckBoxMenuItem("Two");
JMenuItem three = new JCheckBoxMenuItem("Three");
JMenuItem four = new JCheckBoxMenuItem("Four");
menu.add(one);
menu.add(two);
menu.add(three);
menu.add(four);
final JButton button = new JButton("Click me");
button.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
if (!menu.isVisible()) {
Point p = button.getLocationOnScreen();
menu.setInvoker(button);
menu.setLocation((int) p.getX(),
(int) p.getY() + button.getHeight());
menu.setVisible(true);
} else {
menu.setVisible(false);
}
}
});
one.addActionListener(new OpenAction(menu, button));
two.addActionListener(new OpenAction(menu, button));
three.addActionListener(new OpenAction(menu, button));
four.addActionListener(new OpenAction(menu, button));
JFrame frame = new JFrame();
JPanel panel = new JPanel();
panel.add(button);
frame.getContentPane().add(panel);
frame.pack();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
private static class OpenAction implements ActionListener {
private JPopupMenu menu;
private JButton button;
private OpenAction(JPopupMenu menu, JButton button) {
this.menu = menu;
this.button = button;
}
#Override
public void actionPerformed(ActionEvent e) {
menu.show(button, 0, button.getHeight());
}
}
}

There are a few basic problems with creating custom combobox popup content (like a list with multiselection):
1. Default UI suggests JList usage as the content so to change that behavior you will have to change the whole ComboBoxUI
2. You cannot simply change the default combobox list into multiselection one due to the fact that only one value gets "selected" at the end and list has default rollover selection mouse listener, that will make you unable to choose more than one element
So i'd reccomend you to use simple JList instead of combobox or look into using some extended components libraries like JideSoft - they have this component and lots more which you won't be able to quickly create using Swing features.