Swing Hacks

Add Translucence to Menus #12

Chapter 1, Basic JComponents

|

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