Swing Hacks

Size Your Columns to Suit Your JTable’s Contents #21

Chapter 3, Tables and Trees

|

103

HACK

If it does, then we have a usability problem to discuss. By default, the col-

umns of a

JTable are all the same size. For this data, that’s obviously a terri-

ble decision—there’s far too much space reserved for the numbers in the

count column, and not nearly enough in the URL column. So, how are you

going to fix this?

If you said “turn on the horizontal scrollbar,” please close this book, grasp it

with both hands, and firmly smack yourself in the head with it. No, you are

not turning on the horizontal scrollbar! Use the pixels available to you

before you resort to the user-annoying desperation of horizontal scrolling. In

this case, the count column has lots of pixels to spare; you just need to real-

locate this extra space to the URL column.

Resetting Column Widths

What makes programmatic column resizing difficult for many Swing pro-

grammers is that they can’t even find the right methods to use. If all you ever

work with is

JTable (and maybe a few custom cell renderers), you’ll notice

that the JavaDoc for those classes says nothing about column widths. The

problem may be that the

JTable is so generous with helpful methods that

you’d never even notice that it’s made up of

TableColumn objects. Take a

look at that

TableColumn’s JavaDoc, and you’ll find getters and setters for

minimum, maximum, and preferred widths for columns.

As with components that defer to layout managers, the right property to

reset is the preferred width—let the column tell the

JTable how wide it

would like to be, but let the

JTable make the final decision based on the

information available to it (after all, there could be other columns contend-

ing for space, there might not be enough space for all the columns’ pre-

ferred size, etc.).

Given that the key to this hack will be calling

setPreferredWidth( ) on the

TableColumns, the obvious question is: what value should you use for the

preferred width? Here’s a strategy: assuming you already have the table data,

use the width of the widest item in the column. And how do you get that?

By using the table’s cell renderers to actually figure out how big each cell

should be.

ColumnResizer, shown in Example 3-1, has a single static method,

adjustColumnPreferredWidths( ), which takes a JTable as its only argument.

Yes, the name is a mouthful, but it’s helpful to express

exactly what the method does.

104

|

Chapter 3, Tables and Trees

#21 Size Your Columns to Suit Your JTable’s Contents

HACK

For each column in the table, the method goes through all the rows and

renders each cell. It keeps a running tally of the widest component in the

column, and after considering all the rows, it sends this maximum width to

setPreferredWidth( ).

Example 3-2 demonstrates a unit test class to exercise

ColumnResizer.

TestColumnResizer simply puts the JTable into a JFrame, pack( )s it, and shows

it. After a five-second delay, it calls

adjustColumnPreferredWidths() to reset

the preferred column widths, and it

revalidate( )s the table to get a repaint.

Example 3-1. Adjusting column sizes to suit their contents

public class ColumnResizer {

public static void adjustColumnPreferredWidths(JTable table) {

// strategy - get max width for cells in column and

// make that the preferred width

TableColumnModel columnModel = table.getColumnModel( );

for (int col=0; col<table.getColumnCount( ); col++) {

int maxwidth = 0;

for (int row=0; row<table.getRowCount( ); row++) {

TableCellRenderer rend =

table.getCellRenderer(row, col);

Object value = table.getValueAt (row, col);

Component comp =

rend.getTableCellRendererComponent (table,

value,

false,

false,

row,

col);

maxwidth = Math.max (comp.getPreferredSize( ).width,

maxwidth);

} // for row

TableColumn column = columnModel.getColumn (col);

column.setPreferredWidth (maxwidth);

} // for col

}

}

Example 3-2. Testing automatic column sizing

public class TestColumnResizer {

final static Object[][] TABLE_DATA = {

{new Integer(1), "ONJava", "http://www.onjava.com/"},

{new Integer(2), "Joshy's Site", "http://www.joshy.org/"},

{new Integer(3), "Anime Weekend Atlanta", "http://www.awa-con.com/"},

{new Integer(4), "QTJ book",

"http://www.oreilly.com/catalog/quicktimejvaadn/"}

};