Swing Hacks

Add Translucence to Menus #12

Chapter 1, Basic JComponents

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53

HACK

because the JMenu doesn’t draw what we think of as a menu—a list of menu

items that pop up when you click on the menu’s title. The

JMenu actually

only draws the title at the top of a menu. The rest of the menu is drawn by a

JPopupMenu

created as a member of the

JMenu

. Unfortunately this member is

marked

private

, which means you can’t substitute your own

JPopupMenu

subclass for the standard version.

Fortunately there is a way out. Like all Swing components, the menu com-

ponents delegate their actual drawing to a separate set of Look and Feel

classes in the

javax.swing.plaf

package. If you override the right

plaf

classes for the menu items and pop-up menu, then you should be able to

create the desired translucent effect. It just takes a little subclassing.

Make the Custom Menu Item

All MenuItems are implemented by some form of the javax.swing.plaf.

MenuItemUI

class. When creating custom UI classes, it is always best to start

by subclassing something in the

javax.swing.plaf.basic package (in this

case,

BasicMenuItemUI) because it handles most of the heavy lifting for you,

as shown in Example 1-28.

Example 1-28. Extending the basic UI

public class CustomMenuItemUI extends BasicMenuItemUI {

public static ComponentUI createUI(JComponent c) {

return new CustomMenuItemUI( );

}

public void paint(Graphics g, JComponent comp) {

// paint to the buffered image

BufferedImage bufimg = new BufferedImage(

comp.getWidth( ),

comp.getHeight( ),

BufferedImage.TYPE_INT_ARGB);

Graphics2D g2 = bufimg.createGraphics( );

// restore the foreground color in case the superclass needs it

g2.setColor(g.getColor( ));

super.paint(g2,comp);

// do an alpha composite

Graphics2D gx = (Graphics2D) g;

gx.setComposite(AlphaComposite.getInstance(

AlphaComposite.SRC_OVER,0.8f));

gx.drawImage(bufimg,0,0,null);

}

}