Swing Hacks

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Chapter 1, Basic JComponents

#12 Add Translucence to Menus

HACK

No constructor is required because all UI classes have a no-arg constructor

automatically. All UI classes also need a static

createUI( ) method to create a

new instance of the class, as you can see in the example. In the

paint( )

method, instead of drawing on the graphics object passed in, the code cre-

ates a buffered image with the same dimensions as the component, and then

calls

super.paint( )

. This will draw the component onto the buffered image

instead of the screen. Once the painting is done, it can apply a transform and

then draw the image buffer onto the real

Graphics. In this case, the trans-

form is an alpha composite of

0.8. This means that instead of drawing the

buffer as is, it will draw the buffer partially transparent (80% solid, in this

case). This will draw the

bufferedimage into the real graphics with a translu-

cent effect. You can vary the strength of the translucency by modifying the

second parameter to the

AlphaComposite.getInstance( ) method (1 results in

a solid,

0 is totally transparent).

Add a Custom JMenu

If you stopped with just the custom menu items, the menus would seem a

bit translucent, but the rest of the window wouldn’t shine through. This is

because the menu items are inside of another component; in fact, they’re

inside of three! The

JMenu puts all of the menu items inside of a JPopupMenu,

which is placed inside of a

JPanel, and then the whole deal is put in a lay-

ered pane at the top of the frame. The layered pane is already transparent, so

you don’t need to worry about it, but the

JPanel and JPopupMenu are going to

be a problem. Example 1-29 handles the custom UI for these.

Example 1-29. Handling translucence for JPanels and JPopupMenus

public class CustomPopupMenuUI extends BasicPopupMenuUI {

public static ComponentUI createUI(JComponent c) {

return new CustomPopupMenuUI( );

}

public void installUI(JComponent c) {

super.installUI(c);

popupMenu.setOpaque(false);

}

public Popup getPopup(JPopupMenu popup, int x, int y) {

Popup pp = super.getPopup(popup,x,y);

JPanel panel = (JPanel)popup.getParent( );

panel.setOpaque(false);

return pp;

}

}