Swing Hacks

Building a Drop-Down Menu Button #10

Chapter 1, Basic JComponents

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HACK

H A C K

#10

Building a Drop-Down Menu Button

Hack #10

This hack shows how to build a color chooser as a proper drop-down

component. It will behave like

JComboBox but without the extension

headaches of Sun’s version of the class.

Most custom Swing components are created with simple subclasses of the

standard base classes in

javax.swing

. This works fine most of the time, but

every now and then you need to build something where there is no easy

standard component to start with. Even worse, sometimes the obvious

choice for your starting point is a component so convoluted that you can’t

figure out where to start. Still, you’d rather not reimplement the wheel. No,

I’m not talking about

JTree or JTable—I’m referring to the JComboBox. It

seems like such a simple component, but the implementation is fiendishly

complex.

Most large applications use components that feel like the

JComboBox, but do

something entirely different, like select a color or show a history list. A quick

search through the

JComboBox API doesn’t turn up any obvious extension

points. You could customize it with some cell renderers, but if you need a

component that doesn’t show a list of data, you are pretty much out of luck.

The source to

JComboBox is not very helpful either. The work is spread out

over several UI classes in the various Look and Feel (L&F) packages. If you

did customize one of those, your component would look out of place when

used in a different L&F. The only real option is to write your own combo

box, which is pretty easy except for the actual drop-down part. You need to

show a component on top of the others, poking out of the frame occasion-

ally, but without any decorations of its own. It should be just a borderless

floating box. Digging through Swing’s source code reveals the secret ingredi-

ent: a

JWindow.

JWindow is a subclass of Window but not of Frame. This means it has no decora-

tions on the side, and it is hidden from the Dock and Taskbar. This is

exactly what you want from a pop up. Care must be taken when creating it,

however, as you must ensure the window appears only on top of the exist-

ing components, and that it disappears when something else gains focus or

the window moves. Fortunately, you can do all of this with one composite

component and a few event listeners.

DropDownComponent will be a composite of the visible component, a down

arrow trigger button, and the hidden component that will appear in the pop

up. By thinking of your custom component as a composite of existing com-

ponents, you can make it very flexible. Additionally, subclasses must be able

to add different components to make something new out of the same pieces.

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Chapter 1, Basic JComponents

#10 Building a Drop-Down Menu Button

HACK

Example 1-21 is the start of a DropDownComponent class. It extends JComponent

directly and implements both the action and ancestor listener interfaces. It

assembles the visible and drop-down components passed into its construc-

tor with an arrow trigger and the listeners.

The arrow is just a

JButton with a MetalComboBoxIcon. Reusing this arrow lets

the code pick up any Metal Look and Feel customizations. The last line of

the constructor calls another method in the class,

setupLayout( ), to posi-

tion the arrow next to the visible component while letting the component

still grow:

protected void setupLayout( ) {

GridBagLayout gbl = new GridBagLayout( );

GridBagConstraints c = new GridBagConstraints( );

setLayout(gbl);

c.weightx = 1.0; c.weighty = 1.0;

c.gridx = 0; c.gridy = 0;

c.fill = c.BOTH;

gbl.setConstraints(visible_comp,c);

add(visible_comp);

c.weightx = 0;

c.gridx++;

gbl.setConstraints(arrow,c);

add(arrow);

}

Example 1-21. Skeleton for a drop-down combo box

public class DropDownComponent extends JComponent

implements ActionListener, AncestorListener {

protected JComponent drop_down_comp;

protected JComponent visible_comp;

protected JButton arrow;

protected JWindow popup;

public DropDownComponent(JComponent vcomp, JComponent ddcomp) {

drop_down_comp = ddcomp;

visible_comp = vcomp;

arrow = new JButton(new MetalComboBoxIcon( ));

Insets insets = arrow.getMargin( );

arrow.setMargin( new Insets( insets.top, 1, insets.bottom, 1 ) );

arrow.addActionListener(this);

addAncestorListener(this);

setupLayout( );

}

}