Swing Hacks

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Chapter 2, Lists and Combos

#16 Make Different List Items Look Different

HACK

When you click on items in the list, their selection state is shown with

checkboxes, as seen in Figure 2-6.

Notice how the checkboxes are all you need to see what’s selected. In an

early version of this hack, I went out of my way to maintain the usual selec-

tion colors and it looked lousy, probably because there were two competing

metaphors to show the selection: the highlight and the checkbox. Since this

hack totally changes how the JList works, it’s appropriate to radically

change its appearance, too.

H A C K

#16

Make Different List Items Look Different Hack #16

An in-progress download shouldn’t look like a completed one.

What made me love lists again were the OmniWeb browser and (later)

Safari—particularly, their download managers. By way of negative example,

take the download manager for Internet Explorer 5 for Mac...please. This

GUI was a table of filenames, URLs, sizes, etc., with columns not even intel-

ligently resized for their widths. OmniWeb, on the other hand, showed a

running download with a progress bar, and a finished download with the

defaultComp.getListCellRendererComponent (list, value, index,

isSelected, cellHasFocus);

checkbox.setSelected (isSelected);

Component[] comps = getComponents( );

for (int i=0; i<comps.length; i++) {

comps[i].setForeground (listForeground);

comps[i].setBackground (listBackground);

}

return this;

}

}

Figure 2-6. Using the checkbox-metaphor JList

Example 2-8. ListCellRenderer for checkbox-based JList (continued)

Make Different List Items Look Different #16

Chapter 2, Lists and Combos

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71

HACK

file location and file size. Safari goes a step further with context-appropriate

buttons: an X to cancel an in-progress download, a magnifying glass to

locate an already-downloaded file, etc. But it’s the same idea: different things

shouldn’t look the same.

To do this in Swing, you need a hack that goes against everything in all the

other Swing books: you need to stop subclassing

JComponent

when you write

a

ListCellRenderer. Instead, delegate the getListCellRendererComponent( )

call to one of several components, choosing whichever best represents the

item to be rendered.

In fact, the whole tradition of subclassing

JComponent for ListCellRenderers

is a pretty hateful practice because they’re not really used as

Components any-

way! They’re certainly not added to the

JList. Instead, a list cell is rendered

off screen and those pixels are blitted to the

JList. So, provided that what

you return in

getListCellRendererComponent is what you want the cell to

look like, it really doesn’t matter how you get there.

By way of demonstration, this hack shows the items in a given directory

with different layouts, depending on the file type. All of the cells use an icon

on the left, with a name in bold at the top of a two-line layout. However, if

the item is a folder, the bottom line contains a count of the children in that

folder. If the item is a text file—it ends with one of the various extensions

associated with text files (e.g., .txt, .html, .java)—then there’s a different lay-

out that uses two cells on the second line to show file size and word count.

And if the item is determined to be an image file, then a layout with room

for a little two-row image icon on the right is used.

So, there are four different prototypes, and

getListCellRendererComponent( )

needs to choose one, set its fields and highlight colors, and return it.

Example 2-9 is quite long, in part because of the icky

GridBagLayout work

required to make the four prototypes look interesting. This is one time when

I really wish I had a visual Swing GUI builder.

Example 2-9. A JList with multiple cell-rendering layouts

public class PolymorphicJList extends JList {

static Color listForeground, listBackground,

listSelectionForeground, listSelectionBackground;

static {

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

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

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

listSelectionForeground = uid.getColor ("List.selectionForeground");

listSelectionBackground = uid.getColor ("List.selectionBackground");

}