Swing Hacks

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Chapter 2, Lists and Combos

#15 Make JLists Checkable

HACK

The general idea of this pop up can be modified to work in similar ways for

different kinds of searches, such as popping up a list of which field the

search is to be applied to if the list items are complex. An example of this is

a mail program in which the pop up might show the options Subject, To,

From, etc., meaning that the search is limited to finding the specified term in

only the selected field.

H A C K

#15

Make JLists Checkable Hack #15

Avoid losing 50 selections to an unshifted click.

One horrible UI problem is dealing with vast collections of things that need

to be presented to the user and made selectable. If, like me, you’ve ever had

1,000 emails in your inbox, you know what I mean. Worse, what if you pick

a bunch of items to delete, but your finger slips off the key used for multi-

selection (Alt on Windows, Command on the Mac, etc.) and you lose all of

your previous selections? Overriding the native selection behavior can make

this situation somewhat more palatable.

Because a list like this behaves differently than a normal list, it should look

different, too, so I’ve opted for a checkbox metaphor. Each item is shown

with a checkbox, and as you click more items, they get checked, and if you

select an already-checked item, it gets unchecked.

This turns out to be a little harder than expected. I once did

it without

JList, creating my own scrolling layout of JPanels

and faking the list behavior. It turned up a funny Swing bug

because I was using

GridBagLayout for the fake list, and it

started totally bombing out after about 500 items were

added to the list. This was because

GridBagLayout has a bug

where it can’t have more than 512 rows. Considering the bug

(number 4254022 on the Java Bug Parade) was filed in 1999

and is still open, I’m figuring it won’t get fixed by the time

you read this.

The basis of the checkable list is a JList. The tricky part here is that there

isn’t a way (that I’ve found) to steal the mouse clicks from the

JList and

consume them before the normal calls to the

ListSelectionModel are made.

Instead, the strategy is to set up a

ListSelectionListener and just fix every-

thing after

JList has done its thing.

To implement the checkbox functionality, subclass

JList and give it a cus-

tom

ListSelectionListener and a ListCellRenderer. A complete listing is

shown in Example 2-6.

Make JLists Checkable #15

Chapter 2, Lists and Combos

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67

HACK

There are two important sections to consider. The first is the valueChanged( )

method that implements ListSelectionListener. The following list describes

what happens in that method.

Example 2-6. A checkbox-metaphor JList

public class CheckBoxJList extends JList

implements ListSelectionListener {

static Color listForeground, listBackground;

static {

UIDefaults uid = UIManager.getLookAndFeel().getDefaults( );

listForeground = uid.getColor ("List.foreground");

listBackground = uid.getColor ("List.background");

}

HashSet selectionCache = new HashSet( );

int toggleIndex = -1;

boolean toggleWasSelected;

public CheckBoxJList( ) {

super( );

setCellRenderer (new CheckBoxListCellRenderer( ));

addListSelectionListener (this);

}

// valueChanged( ) listing below

public static void main (String[] args) {

JList list = new CheckBoxJList ( );

DefaultListModel defModel = new DefaultListModel( );

list.setModel (defModel);

String[] listItems = {

"Chris", "Joshua", "Daniel", "Michael",

"Don", "Kimi", "Kelly", "Keagan"

};

Iterator it = Arrays.asList(listItems).iterator( );

while (it.hasNext( ))

defModel.addElement (it.next( ));

// show list

JScrollPane scroller =

new JScrollPane (list,

ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS,

ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

JFrame frame = new JFrame ("Checkbox JList");

frame.getContentPane( ).add (scroller);

frame.pack( );

frame.setVisible(true);

}

}