JMenu Construction problem

[Balasubramanian Chandrasekaran]

Hi folks,

I have a problem with JMenu construction.

Have a look at this code

 
/**
 * @(#)MenuAdd.java
 *
 *
 * @author BALASUBRAMANIAN C
 * @version 1.00 2008/4/17
 */
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JMenuBar;
import javax.swing.JFrame;
import javax.swing.JPopupMenu;
import java.awt.Dimension;
 
public class MenuAdd extends JFrame{
        
    private static MenuAdd m_menuAdd = new MenuAdd();
    private JMenu fileMenu;
    private JMenuBar menuBar = new JMenuBar();
    private JMenuItem menuItem;
    private JPopupMenu popMenu = new JPopupMenu();
    
    public void construct()
    {
    fileMenu = new JMenu("File");
    menuItem = new JMenuItem("New");
    fileMenu.add(menuItem);
    [B]popMenu.add(menuItem);[/B]
    menuBar.add(fileMenu);
    }
    
    public void display()
    {
    setTitle("Hello");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(new Dimension(300,300));
        setJMenuBar(menuBar);
        setVisible(true);
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        m_menuAdd.construct();
        m_menuAdd.display();
    }
}
 

If i uncomment popMenu.add(menuItem) line then only i am able to view "New" menuitem under the "File" menu.

Why is this behavior?
Is there a restriction that we can add component only once. Can anyone explain me this behaviour

I just tried one more implementation in which i tried to add same menuitem on different menu but in that one also i am able to view menuitem on the second menu only

Code which i tried for that implementation is

 
/**
 * @(#)MenuAdd.java
 *
 *
 * @author BALASUBRAMANIAN C
 * @version 1.00 2008/4/17
 */
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JMenuBar;
import javax.swing.JFrame;
import java.awt.Dimension;
 
public class MenuAdd extends JFrame{
        
    private static MenuAdd m_menuAdd = new MenuAdd();
    private JMenu fileMenu;
    private JMenu editMenu;
    private JMenuBar menuBar = new JMenuBar();
    private JMenuItem menuItem;
    
    public void construct()
    {
    fileMenu = new JMenu("File");
    editMenu = new JMenu("Edit");
    menuItem = new JMenuItem("New");
    fileMenu.add(menuItem);
    editMenu.add(menuItem);
    menuBar.add(fileMenu);
    menuBar.add(editMenu);
    }
    
    public void display()
    {
    setTitle("Hello");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(new Dimension(300,300));
        setJMenuBar(menuBar);
        setVisible(true);
    }
    
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        m_menuAdd.construct();
        m_menuAdd.display();
    }
}
 

Waiting for your reply......

[Brian Cole]

Originally posted by Balasubramanian Chandrasekaran:

Is there a restriction that we can add component only once. Can anyone explain me this behaviour

Yes, there is exactly this restriction. If you try to add a Component
to a second Container, it will automatically be removed from the first
Container first. There are good reasons for this, but they are easiest
to understand if you think of text fields and buttons instead of menu
items. Menu items do behave the same as other Components, though.

One thing you can try is instead of attempting to add a JMenuItem multiple
places, instead add an Action to them. An Action isn't a Component, but
can be added directly to a JMenu (in which case a JMenuItem is created
for you behind the scenes).

I just tried one more implementation in which i tried to add same menuitem on different menu but in that one also i am able to view menuitem on the second menu only

Code which i tried for that implementation is

    private JMenu fileMenu;
    private JMenu editMenu;
    private JMenuItem menuItem;
    
    public void construct()
    {
    fileMenu = new JMenu("File");
    editMenu = new JMenu("Edit");
    menuItem = new JMenuItem("New");
    fileMenu.add(menuItem);
    editMenu.add(menuItem);

But you're still adding menuItem to two different Containers. It should
appear correctly if you just do:

    fileMenu = new JMenu("File");
    editMenu = new JMenu("Edit");
    //menuItem = new JMenuItem("New");
    fileMenu.add(new JMenuItem("New"));
    editMenu.add(new JMenuItem("New"));

If you want to take my Action suggestion it would look something like:

    private JMenu fileMenu;
    private JMenu editMenu;
    private Action newAction;
    
    public void construct()
    {
    fileMenu = new JMenu("File");
    editMenu = new JMenu("Edit");
    newAction = new AbstractAction("New") {
    public void actionPerformed(ActionEvent ae) {
    // do something...
    }
    };
    fileMenu.add(newAction);
    editMenu.add(newAction);

[ April 17, 2008: Message edited by: Brian Cole ]

[Balasubramanian Chandrasekaran]

Thanks Brian,

I just thought of reusing the same object.Can you explain me why that restriction is there (in detail).
In the mean while i will try to implement your Action on my code.
[ April 17, 2008: Message edited by: Balasubramanian Chandrasekaran ]

[Brian Cole]

Originally posted by Balasubramanian Chandrasekaran:
I just thought of reusing the same object.Can you explain me why that restriction is there (in detail).

We discussed this a few months ago in another thread.

If you just want to be convinced that it is true, take a look at the
source for the Container.add() methods. They eventually delegate
to Container.addImpl(), which does this:

if (comp.parent != null) {
comp.parent.remove(comp);
...
}

If you want to understand the why of this, consider that an instance of
Component has a number of properties that can be accessed with methods
such as getBounds(), getLocation(), getX(), getY(), isFocusOwner(), and
getParent(). How would you suggest these methods be implemented for
Components that have somehow been added to more than one Container?
[ April 17, 2008: Message edited by: Brian Cole ]

[Balasubramanian Chandrasekaran]

Thanks again,
Now i got the point.