Swing Hacks

Add Translucence to Menus #12

Chapter 1, Basic JComponents

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55

HACK

The custom pop-up menu UI used here is similar to the CustomMenuItemUI

(from Example 1-28). It has a static create UI menu and no constructor. The

pop-up menu is already stored as a protected member of the

BasicPopupMenuUI

parent class, so I can access it easily. The

installUI( )

method is called right after the

JPopupMenu

is created, so this is the best place

to put a call to

setOpaque(false)

. For most L&Fs, this will make the compo-

nent transparent.

That takes care of the pop-up menu, but what about the parent

JPanel? The

JPanel

is created and initialized deep within the

javax.swing.PopupFactory

class, so it’s pretty well out of reach. This is one place where having access

to the JRE source code is invaluable. Without that, this entire hack would

have been impossible to figure out. Fortunately, we have access to the fin-

ished

JPopupMenu from within the getPopup method. I overrode that to call

the superclass and then grab the newly minted parent of the pop-up menu

and cast it to a

JPanel. Now, I can finally set it to be transparent, too.

Test It Out

With your two custom UI classes in place, test them out with Example 1-30,

which shows a frame containing two sets of menus and a few components.

Before creating any components, the program installed the custom UI

classes with two calls to

UIManager.put( ).

Any time you want to override part of a L&F, you can use

UIManager.put( ).

Example 1-30. Testing out the translucent menus

public class MenuTest {

public static void main(String[] args) throws Exception {

UIManager.put("PopupMenuUI","CustomPopupMenuUI");

UIManager.put("MenuItemUI","CustomMenuItemUI");

JFrame frame = new JFrame( );

JMenuBar mb = new JMenuBar( );

frame.setJMenuBar(mb);

JMenu menu = new JMenu("File");

mb.add(menu);

menu.add(new JMenuItem("Open"));

menu.add(new JMenuItem("Save"));

menu.add(new JMenuItem("Close"));

menu.add(new JMenuItem("Exit"));

menu = new JMenu("Edit");

mb.add(menu);

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Chapter 1, Basic JComponents

#12 Add Translucence to Menus

HACK

With all of the code in place, you can compile it and get something that

looks like Figure 1-30.

One bug you will notice is that after you open the menu and start moving the

cursor between menu items, the background won’t shine through anymore.

This is because Swing, in an effort to speed up the UI, only repaints the parts

it knows have changed. It repaints the menu item, but not the frame con-

tents below (the button and label, in this case) because it thinks they are

obscured by the menu item. Of course, the menu item is translucent, so the

components should shine through, but Swing doesn’t know that. To fix the

menu.add(new JMenuItem("Cut"));

menu.add(new JMenuItem("Copy"));

menu.add(new JMenuItem("Paste"));

menu.add(new JMenuItem("Paste Special.."));

frame.getContentPane().setLayout(new BorderLayout( ));

frame.getContentPane( ).add("North",new JButton("Button"));

frame.getContentPane( ).add("Center",new JLabel("a label"));

frame.getContentPane( ).add("South",new JCheckBox("checkbox"));

frame.pack( );

frame.setSize(200,150);

frame.show( );

}

}

Figure 1-30. Translucent menu

Example 1-30. Testing out the translucent menus (continued)