Swing components that are composed of other components behave inconsistently compared to simple Swing components. An example of a compound component is JComboBox. JComboBox has an edit field and an arrow button contained within itself. The edit field and the button are children of the JComboBox.

Compound components have the following problems:
1) They confuse event handling by having multiple sources.
2) They stop tool tips from working properly.
3) Setting cursors on them will cause strange behavior.
4) They are very difficult to initialize.

Problem #1: They confuse event handling by having multiple sources.
People have been registering mouse and keyboard listeners with JComboBoxes. When the user clicks on the arrow button the events come from the arrow button, not the JComboBox. This has been confusing developers. They expect event handling to be the same for all Swing components regardless of how they were implemented. In fact, whether or not a component is a compound component is totally dependent on the UI delegate. If one L&F doesn't use children to create the component then a developer can't rely on the fact that it's compound or not when they decide to attach listeners to it.

Problem #2: They stop tool tips from working properly.
Tool tips suffer from a similar problem to event handling. If a developer puts a tool tip on an editable JComboBox, the tool tip will only appear if the cursor is over the combo box but not over the arrow button or the edit field.

Problem #3: Setting cursors on them will cause strange behavior.
Trying to set a cursor on an editable combo box will result in behavior similar to tool tips. The cursor will only appear if the cursor is over the combo box but not over the arrow button or the edit field.

Problem #4: They are very difficult to initialize
In order for a compound component to be properly initialized during a Look & Feel switch, an invokeLater() call must be used to initialize the child components. The way that component-trees are updated during a L&F switch (using SwingUtilities.updateComponetTreeUI) causes this to be a problem. I'll use JComboBox as an example:

1. updateComponetTreeUI() tells the JComboBox to update its UI
2. A ComboBoxUI subclass gets created
3. The ComboBoxUI adds some child components to the JComboBox (like the editor)
4. The ComboBoxUI initializes the child components with fonts and colors
5. updateComponentTreeUI() asks the JComboBox's children to update their UIs
6. JComboBox's children now make new UI delegates
7. The new UI delegates re-initialize the child components with values from the defaults table.

The ComboBoxUI set values in the children but they were later wiped out because the children were asked to create new delegates who re-initialized the children.

tom.santos@eng 1998-06-01

Name: krT82822 Date: 06/28/99


setEnabled is inconsistant between JComponents.
For most components(All the ones I checked), Components will still generate events when setEnabled(false) is called
This does not occur for JComboBox though.

eg:
If you run this program as is, clicking will not generate a mouse event.
Change variable0 to any other component(eg: a JButton), and it will

(test case follows at end, but the next section summarizes additional comments from the user received 6/28/99 [6/29 in user's timezone]):

> Subject: Re: [Re: (Review ID: 53321) setEnabled does not work correctly with JComboBox]
> Date: 29 Jun 99 09:27:21 WST
> From: Daniel Fletcher <###@###.###>


G'day
...
Basically I believe that when the JComboBox is disabled, it should begreyed
out, and clicking shouldnt change it in any way, but the mouseEvents should
still be generated and sent to any listeners.

My reason for wanting this?
I was making a GUI builder called
Korfe(http://www.javalobby.org/jfa/projects/korfe.html)
which allowed you to move and drag components around.

For all components except JCombobox, I was able to disable the Component, and
then let the user drag it around when they clicked in it etc.

For JCombobox once the component was disabled, clicking in it would not
generate mouse events, so I couldnt catch them, and thus it couldnt be
dragged.

I think the correct behaviour would be the same as the other components.
Consistency is defintely the way to go, so if you do decide to continue not
generating mouse events, you should do it for all other components as
well(Even though it will totally break my application :)

Daniel

----------------------



------------------------
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

class Test extends JPanel implements MouseListener
{

public Test()
{
super();
setupGUI();
addMouseListener(this);
variable0.addMouseListener(this);
variable0.setEnabled(false);
}

//Call this method to setup the GUI
public void setupGUI()
{
BorderLayout layout1 = new BorderLayout();
setLayout(layout1);
variable0 = new JComboBox();
add(variable0 , "Center");

}

public static void main(String args[])
{
JFrame f=new JFrame();
Test t=new Test();
f.getContentPane().setLayout(new BorderLayout());
f.getContentPane().add(t,BorderLayout.CENTER);
f.setBounds(0,0,300,200);
f.setVisible(true);
}

public void mouseClicked(MouseEvent e)
{
}

public void mouseEntered(MouseEvent e)
{
}

public void mouseExited(MouseEvent e)
{
}

public void mousePressed(MouseEvent e)
{
System.out.println("Mouse pressed");
}

public void mouseReleased(MouseEvent e)
{
}

//Variables
private JComponent variable0;
}
(Review ID: 53321)
======================================================================

Name: rlT66838 Date: 09/28/99


Since the JDK 1.1.8 is impossible to intercept the keyboard event in the JComboBox component.

see the example :
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MyComboBox extends JFrame implements KeyListener
{
    public MyComboBox()
    {
        super("MyComboBox");
        getContentPane().setLayout(new GridLayout(1, 2));
        addWindowListener(new WindowAdapter()
        {
           public void windowClosing(WindowEvent e)
           {
               System.exit(0);
           }
        });

        JComboBox myComboBox = new JComboBox();
        myComboBox.addKeyListener(this);
        myComboBox.setEditable(true);
        getContentPane().add(myComboBox);
        
pack();
setVisible(true);
    }

    public static void main(String arg[]) {new MyComboBox();}
    public void keyPressed(KeyEvent e) {System.out.println("keyPressed");}
    public void keyReleased(KeyEvent e) {System.out.println("keyReleased");}
    public void keyTyped(KeyEvent e) {System.out.println("keyTyped");}
}
(Review ID: 95842)
======================================================================