How to specify JTable's column widths
Written by Don
Young , InPowerSoft, February 2006
Introduction
Very often, when you construct a JTable and bind it to a ADF view object,
many attributes of view object will be displayed in columns of a JTable.
If you construct a JTable with automatic column sizing, then when the
JTable is resized, its columns will also be resized accordingly. But how
would you control the relative width proportion of each column within
a JTable. This article shows a simple API to achieve this effect. With
this function, you can limit the relative width of each column in proportion
to other columns and this proportion is kept automatically even during
the JTable resizing process.
Step #A
Ideally, we like a function such that we can do the followings:
18 DbTable jobTable = new DbTable(getFormParameters()); 19
20 jobTable.createBinding(block, new String[]{"JobTitle", "MinSalary", "MaxSalary"});
21
22 jobTable.setPreferredColumnWidths(new double[]{0.4, 0.2, 0.2});
Line 18 creates a JTable in this case is a derived version called DbTable.
We specify in line 20 the column bindings to the view object. Line 22
is the API of interest and we specify their relative widths. The source
code is taken from the demo example at the InPowerSoft download
page. The end result will look like the following:
Step #B
Let's dissect the function we will be building:
134 public void setPreferredColumnWidths(double[] percentages)
135 {
136 Dimension tableDim = this.getPreferredSize();
137
138 double total = 0;
139 for(int i = 0; i < getColumnModel().getColumnCount(); i++)
140 total += percentages[i];
141
142
143 for(int i = 0; i < getColumnModel().getColumnCount(); i++)
144 {
145 TableColumn column = getColumnModel().getColumn(i);
146 column.setPreferredWidth((int) (tableDim.width * (percentages[i] / total)));
147 }
148 }
This function cannot be more simpler. The principle is to set the preferred
size at each column according to the relative proportion specified at the
percentages array passed in. Line 136 give us the preferred
size of the table as it is determined by the layout manager of this JTable.
Line 138 to 140 give us the sum of each column's proportional value. We
then go through another pass and compute each column's relative proportion
and use the table's preferred width to set each column's preferred width.
Note, the value in percentages array does not have to add
up to 1.0. As a matter of fact, you can specify an array of any floating
point number and this function will work properly. This feature is handy
because many often you will find yourself adding and deleting columns as
a frequent exercise during the coding process and if you have to calibrate
the sum to 1.0, it will be a bit tedious.
Step #C
One obvious improvement can be made to this function. We do not test
for negative floating point values. Instead of disallowing negative floating
point values, we may use negative floating point values to specify the
exact number of pixels a column can paint instead of its relative width
in relation to others. This is left as an exercise to your imagination.
You must not call this function before you create your columns or before
create binding to the view object that creates the columns. Note also
by setting the preferred size on the columns, you only specify the preference
of column width but the layout managers will determine how is best to
show each column according to a variety of other factors. Generally, you
will get very close to what you want.
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