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<html>
<title>Java Excel API Tutorial</title>
<link rel="stylesheet" href="jxl.css" />
<body>
<p class="bighead">
Java Excel API Tutorial
</p>
<p class="head">
Contents
</p>
<a href="#introduction">Introduction</a>
<br />
<a href="#reading">Reading a spreadsheet</a>
<br />
<a href="#writing">Writing a spreadsheet</a>
<br />
<a href="#fundamentals">Fundamentals</a>
<br />
<a href="#formatting cells">Adding format information</a>
<br />
<a href="#formatting numbers">Formatting numbers</a>
<br />
<a href="#formatting dates">Formatting dates</a>
<br />
<a href="#copying and modifying">Copying and modifying a spreadsheet</a>
<br />
<a href="#demo programs">Demo & Test programs</a>
<br />
<a href="#faq">Frequently Asked Questions</a>
<br />
<a href="#outofmemory">java.lang.OutOfMemory Exception</a>
<br />
<a href="#compiling">Compiling</a>
<br />
<a href="#uploading">Uploading a Spreadsheet</a>
<br />
<a href="#charts">Support for charts, macros and images</a>
<br />
<a href="#dates">Date display</a>
<br />
<a href="#multiple workbooks">Cell formats across multiple workbooks</a>
<br />
<a href="#cross sheet formulas">Cross sheet formulas</a>
<br />
<a name="introduction" />
<p class="head">
Introduction
</p>
<p>
The information presented in this tutorial is intended as
a brief overview as to how
JExcelApi may be used to read and write data
in Excel format. The tutorial is by no means exhaustive, and if
something is not described here, it does not mean that it cannot
be done. The reader is encouraged to examine the API documentation
and the sample code (particularly Write.java and ReadWrite.java)
in order to gain a more complete understanding of the capabilities
and limitations of the software.
</p>
<a name="reading" />
<p class="head">
Reading Spreadsheets
</p>
<p>
JExcelApi can read an Excel spreadsheet from a file stored on the local
filesystem or from some input stream.
The first step when reading a spreadsheet from a file or input stream
is to create a Workbook. The code fragment below illustrates
creating a workbook from a file on the local filesystem.
<br /><br />
<blockquote>
<a class="technical">
import java.io.File;
<br />
import java.util.Date;
<br />
import jxl.*;
<br />
<br />
...
<br />
<br />
Workbook workbook = Workbook.getWorkbook(new File("myfile.xls"));
</a>
</blockquote>
(NOTE: when creating a spreadsheet from a ServletInputStream
you must remove the HTTP header information before creating the
Workbook object.)
<br /><br />
Once you have accessed the workbook, you can use this to access
the individual sheets. These are zero indexed - the first sheet being
0, the second sheet being 1, and so on. (You can also use the
API to retrieve a sheet by name).
<br />
<br />
<blockquote>
<a class ="technical">
Sheet sheet = workbook.getSheet(0);
</a>
</blockquote>
Once you have a sheet, you can then start accessing the cells. You
can retrieve the cell's contents as a string by using the convenience
method getContents(). In the example code below, A1 is a text
cell, B2 is numerical value and C2 is a date. The contents of these
cells may be accessed as follows
<br />
<br />
<blockquote>
<a class="technical">
Cell a1 = sheet.getCell(0,0);
<br />
Cell b2 = sheet.getCell(1,1);
<br />
Cell c2 = sheet.getCell(2,1);
<br />
<br />
String stringa1 = a1.getContents();
<br />
String stringb2 = b2.getContents();
<br />
String stringc2 = c2.getContents();
<br />
<br />
// Do stuff with the strings etc
<br />
...
</a>
</blockquote>
The demo programs CSV.java and XML.java use the convenience method
getContents() in order to output the spreadsheet contents.
<br />
<br />
However if it is required to access the cell's contents as the exact
type ie. as a numerical value or as a date, then the retrieved Cell
must be cast to the correct type and the appropriate methods called.
The section of code below illustrates how JExcelApi may be used
to retrieve a genuine java double and java.util.Date object from an Excel
spreadsheet. For completeness the label is also cast to it's correct
type, although in practice this makes no difference. The example
also illustrates
how to verify that cell is of the expected type - this can be useful
when validating that the spreadsheet has cells in the correct place.
<br />
<br />
<blockquote>
<a class="technical">
String stringa1 = null;
<br />
double numberb2 = 0;
<br />
Date datec2 = null;
<br />
<br />
Cell a1 = sheet.getCell(0,0);
<br />
Cell b2 = sheet.getCell(1,1);
<br />
Cell c2 = sheet.getCell(2,1);
<br />
<br />
if (a1.getType() == CellType.LABEL)
<br />
{
<br />
LabelCell lc = (LabelCell) a1;
<br />
stringa1 = lc.getString();
<br />
}
<br />
<br />
if (b2.getType() == CellType.NUMBER)
<br />
{
<br />
NumberCell nc = (NumberCell) b2;
<br />
numberb2 = nc.getValue();
<br />
}
<br />
<br />
if (c2.getType() == CellType.DATE)
<br />
{
<br />
DateCell dc = (DateCell) c2;
<br />
datec2 = dc.getDate();
<br />
}
<br />
<br />
// Do stuff with dates and doubles
<br />
...
</a>
</blockquote>
When you have finished processing all the cells, use the close()
method.
This frees up any allocated memory used when reading spreadsheets and
is particularly important when reading large spreadsheets.
<br />
<br />
<blockquote>
<a class ="technical">
// Finished - close the workbook and free up memory
<br />
workbook.close();
</a>
</blockquote>
</p>
<br />
<a name="writing" />
<p class="head">
Writing Spreadsheets
</p>
<a name="fundamentals" />
<p class="smallhead">
Fundamentals
</p>
<p>
This section describes how to write out simple spreadsheet data without
any formatting information, such as fonts or decimal places.
<br /><br />
Similarly to reading a spreadsheet, the first step is to create a
writable workbook using the factory method on the Workbook class.
<br /><br />
<blockquote>
<a class="technical">
import java.io.File;
<br />
import java.util.Date;
<br />
import jxl.*;
<br />
import jxl.write.*;
<br />
<br />
...
<br />
<br />
WritableWorkbook workbook = Workbook.createWorkbook(new File("output.xls"));
</a>
</blockquote>
This creates the workbook object. The generated file will be located
in the current working directory and will be called "output.xls".
The API can also be used to send the workbook directly to an output
stream eg. from a web server to the user's browser. If the
HTTP header is set correctly, then this will launch Excel and display
the generated spreadsheet.
<br />
<br />
The next stage is to create sheets for the workbook. Again, this is
a factory method, which takes the name of the sheet and the position it
will occupy in the workbook. The code fragment below creates a sheet
called "First Sheet" at the first position.
<br />
<br />
<blockquote>
<a class="technical">
WritableSheet sheet = workbook.createSheet("First Sheet", 0);
</a>
</blockquote>
Now all that remains is to add the cells into the worksheet. This is
simply a matter of instantiating cell objects and adding them to the
sheet. The following code fragment puts a label in cell A3, and the
number 3.14159 in cell D5.
<br />
<br />
<blockquote>
<a class="technical">
Label label = new Label(0, 2, "A label record");
<br />
sheet.addCell(label);
<br />
<br />
Number number = new Number(3, 4, 3.1459);
<br />
sheet.addCell(number);
</a>
</blockquote>
There are a couple of points to note here. Firstly, the cell's
location in the sheet is specified as part of the constructor
information. Once created, it is not possible to change a cell's
location, although the cell's contents may be altered.
<br />
<br />
The other point to note is that the cell's location is specified
as (column, row). Both are zero indexed integer values - A1 being
represented by (0,0), B1 by (1,0), A2 by (0,1) and so on.
<br />
<br />
Once you have finished adding sheets and cells to the workbook, you
call write() on the workbook, and then close the file. This final step
generates the output file (output.xls in this case)
which may be read by Excel. If you call close() without calling write()
first, a completely empty file will be generated.
<br />
<br />
<blockquote>
<a class="technical">
...
<br />
// All sheets and cells added. Now write out the workbook
<br />
workbook.write();
<br />
workbook.close();
</a>
</blockquote>
</p>
<br />
<a name="formatting cells" />
<p class="smallhead">
Adding Format Information
</p>
<p>
The previous section illustrates the fundamentals of generating an
Excel compatible spreadsheet using the JExcelApi. However,
as it stands Excel will render the data in
the default font, and will display the numbers to 3 decimal places. In
order to supply formatting information to Excel, we must make use
of the overloaded constructor, which takes an additional object
containing the cell's formatting information (both the font and
the style).
<br />
<br />
The code fragment below illustrates creating a label cell for an arial
10 point font.
<blockquote>
<a class="technical">
// Create a cell format for Arial 10 point font
<br />
WritableFont arial10font = new WritableFont(WritableFont.ARIAL, 10);
<br />
WritableCellFormat arial10format = new WritableCellFormat
(arial10font);
<br />
<br />
// Create the label, specifying content and format
<br />
Label label2 = new Label(1,0, "Arial 10 point label",
arial10format);
<br />
sheet.addCell(label2);
</a>
</blockquote>
Cell formats objects are shared, so many cells may use the same format
object, eg.
<br />
<br />
<blockquote>
<a class="technical">
Label label3 = new Label(2, 0, "Another Arial 10 point label",
arial10format);
<br />
sheet.addCell(label3);
</a>
</blockquote>
This creates another label, with the same format, in cell C1.
<br />
<br />
Because cell formats are shared, it is not possible to change the
contents of a cell format object. (If this were permitted, then
changing the contents of the object could have unforeseen
repurcussions on the look of the rest of the workbook). In order
to change the way a particular cell is displayed,
the API does allow you to assign a new format to an individual cell.
<br /><br />
The constructors for the WritableFont object have many overloads.
By way of
example, the code fragment below creates a label in 16 point Times, bold
italic and assigns it to position D1.
<blockquote>
<a class="technical">
// Create a cell format for Times 16, bold and italic
<br />
WritableFont times16font = new WritableFont(WritableFont.TIMES, 16, WritableFont.BOLD, true);
<br />
WritableCellFormat times16format = new WritableCellFormat
(times16font);
<br />
<br />
// Create the label, specifying content and format
<br />
Label label4 = new Label(3,0, "Times 16 bold italic label",
times16format);
<br />
sheet.addCell(label4);
</a>
</blockquote>
</p>
<br />
<a name="formatting numbers" />
<p class="smallhead">
Formatting Numbers
</p>
<p>
Number formatting information may be passed to the cell format object
by a similar mechanism to that described for fonts.
<br />
<br />
A variety of predefined number formats are defined statically.
These may be used to format numerical values as follows:
<blockquote>
<a class="technical">
WritableCellFormat integerFormat = new WritableCellFormat
(NumberFormats.INTEGER);
<br />
Number number2 = new Number(0, 4, 3.141519, integerFormat);
<br />
sheet.addCell(number2);
<br />
<br />
WritableCellFormat floatFormat = new WritableCellFormat
(NumberFormats.FLOAT);
<br />
Number number3 = new Number(1, 4, 3.141519, floatFormat);
<br />
sheet.addCell(number3);
</a>
</blockquote>
The above code inserts the value 3.14159 into cells A5 and B5, using the
preset integer and floating points format respectively. When Excel
renders these cells, A5 will display as "3" and B5 will display as
"3.14", even though both cells contain the same floating point
value.
<br /><br />
It's possible for a user to define their own number formats, by
passing in a number format string. The string passed in should be
in the same format as that used by the java.text.DecimalFormat
class. To format a number to display up to five decimal places
in cell C5, the following code fragment may be used:
<blockquote>
<a class="technical">
NumberFormat fivedps = new NumberFormat("#.#####");
<br />
WritableCellFormat fivedpsFormat = new
WritableCellFormat(fivedps);
<br />
Number number4 = new Number(2, 4, 3.141519, fivedpsFormat);
<br />
sheet.addCell(number4);
</a>
</blockquote>
It is, of course, also possible to specify font information as
well eg. to display the same value in the 16 point times bold font
defined earlier we can write
<blockquote>
<a class="technical">
WritableCellFormat fivedpsFontFormat = new WritableCellFormat
(times16font, fivedps);
<br />
Number number5 = new Number(3, 4, 3.141519, fivedpsFontFormat);
<br />
sheet.addCell(number5);
</a>
</blockquote>
</p>
<br />
<a name="formatting dates" />
<p class="smallhead">
Formatting Dates
</p>
<p>
Dates are handled similarly to numbers, taking in a format compatible
with that used by the java.text.SimpleDateFormat class.
In addition, several predefined date formats are specified in
the jxl.write.DateFormat class.
<br />
<br />
As a brief example, the below code fragment illustrates placing
the current date and time in cell A7 using a custom format:
<br />
<br />
<blockquote>
<a class="technical">
// Get the current date and time from the Calendar object
<br />
Date now = Calendar.getInstance().getTime();
<br />
DateFormat customDateFormat = new DateFormat
("dd MMM yyyy hh:mm:ss");
<br />
WritableCellFormat dateFormat = new WritableCellFormat
(customDateFormat);
<br />
DateTime dateCell = new DateTime(0, 6, now, dateFormat);
<br />
sheet.addCell(dateCell);
<br />
</a>
</blockquote>
As with numbers, font information may be used to display the date text
by using the overloaded constructors on WritableCellFormat.
<br />
<br />
For a more extensive example of writing spreadsheets, the demonstration
program Write.java should be studied. In addition to the functionality
described above, this program tests out a
variety of cell, formatting and font options, as well as displaying
cells with different background and foreground colours, shading and
boundaries.
</p>
<a name="copying and modifying" />
<p class="head">
Copying and Modifying Spreadsheets
</p>
<p>
This section describes the scenario where a spreadsheet is read in,
it's contents altered in some way and the modified spreadsheet
written out.
<br />
<br />
The first stage is to read in the spreadsheet in the normal way:
<blockquote>
<a class="technical">
import java.io.File;
<br />
import java.util.Date;
<br />
import jxl.*;
<br />
import jxl.write.*;
<br />
<br />
...
<br />
<br />
Workbook workbook = Workbook.getWorkbook(new File("myfile.xls"));
</a>
</blockquote>
This creates a readable spreadsheet. To obtain a writable version of
this spreadsheet, a copy must be made, as follows:
<blockquote>
<a class="technical">
WritableWorkbook copy = Workbook.createWorkbook(new File("output.xls"), workbook);
</a>
</blockquote>
The API functions this way is for reasons of
read efficiency (since this is the primary use of the API). In order
to improve performance, data which relates to output information (eg.
all the formatting information such as fonts) is not interpreted when
the spreadsheet is read, since
this is superfluous when interrogating the
raw data values. However, if we need to modify this spreadsheet
a handle to the various write interfaces is needed, which can be obtained
using the copy method above. This copies
the information that has already been read in as well as performing the
additional processing to interpret the fields that are necessary to
for writing spreadsheets. The disadvantage of this
read-optimized strategy is that we have two spreadsheets held in memory
rather than just one, thus doubling the memory requirements. For this
reason copying and modifying large spreadsheets can be
expensive in terms of processing and memory.
<br />
<br />
Once we have a writable interface to the workbook, we may retrieve
and modify cells. The following code fragment illustrates how
to modify the contents of a label cell located in cell B3 in sheet 2
of the workbook.
<blockquote>
<a class="technical">
WritableSheet sheet2 = copy.getSheet(1);
<br />
WritableCell cell = sheet2.getWritableCell(1, 2);
<br />
<br />
if (cell.getType() == CellType.LABEL)
<br />
{
<br />
Label l = (Label) cell;
<br />
l.setString("modified cell");
<br />
}
<br />
</a>
</blockquote>
There is no need to call the add() method on the sheet, since the
cell is already present on the sheet. The contents of numerical
and date cells may be modified in a similar way, by using the
setValue() and setDate() methods respectively.
<br />
<br />
Although cell formats are immutable, the contents of a cell may be
displayed differently by assigning a different format object to the cell.
The following code fragment illustrates changing the format of numerical
cell (in position C5) so that the contents will be displayed to
five decimal places.
<blockquote>
<a class="technical">
WritableSheet sheet2 = copy.getSheet(1);
<br />
WritableCell cell = sheet2.getWritableCell(2, 4);
<br />
<br />
NumberFormat fivedps = new NumberFormat("#.#####");
<br />
WritableCellFormat cellFormat = new WritableCellFormat(fivedps);
<br />
cell.setFormat(cellFormat);
</a>
</blockquote>
Since the copy of the workbook is an ordinary writable workbook, new
cells may be added to the sheet, thus:
<blockquote>
<a class="technical">
Label label = new Label(0, 2, "New label record");
<br />
sheet2.addCell(label);
<br />
<br />
Number number = new Number(3, 4, 3.1459);
<br />
sheet2.addCell(number);
</a>
</blockquote>
As before, once the modifications are complete, the workbook must
be written out and closed.
<blockquote>
<a class="technical">
...
<br />
// All cells modified/added. Now write out the workbook
<br />
copy.write();
<br />
copy.close();
</a>
</blockquote>
The demo program contained in the source file ReadWrite.java may be
studied as a more exhaustive example of how spreadsheets
may be modified. This demo program copies the
spreadsheet passed in on the command line; if the spreadsheet
to be copied is the example spreadsheet, jxlrwtest.xls, located
in the current directory, then certain
modifications are carried out. DO NOT MODIFY THE EXAMPLE SPREADSHEET,
otherwise the demo program will not work.
</p>
<br /><br />
<a name="demo programs" />
<p class="head">
Demonstration and Test Programs
</p>
<p>
JExcelApi comes with a raft of demonstration and test programs contained
in the package jxl.demo. These may be accessed from the command line
as follows
<blockquote>
<a class="technical">
java -jar jxl.jar -csv spreadsheet.xls
</a>
</blockquote>
Reads spreadsheet.xls and writes out the corresponding csv data to
the standard output. The -csv option is the default and may omitted
<blockquote>
<a class="technical">
java -jar jxl.jar -xml spreadsheet.xls
</a>
</blockquote>
Reads spreadsheet.xls and writes out the corresponding cell data to
the standard output as XML.
<blockquote>
<a class="technical">
java -jar jxl.jar -xml -format spreadsheet.xls
</a>
</blockquote>
As above, but includes formatting information (font, number formats etc)
in the generated XML
<blockquote>
<a class="technical">
java -jar jxl.jar -formulas spreadsheet.xls
</a>
</blockquote>
Reads spreadsheet.xls and displays all the formulas contained in that
sheet.
<blockquote>
<a class="technical">
java -jar jxl.jar -write test.xls
</a>
</blockquote>
Generates a sample test spreadsheet called test.xls
<blockquote>
<a class="technical">
java -jar jxl.jar -rw in.xls out.xls
</a>
</blockquote>
Reads in.xls, copies it and generates a file called out.xls. If the
spreadsheet passed in is a special sample spreadsheet called
jxlrwtest.xls, then this demo will modify specific cells in the
copy, out.xls.
</p>
<a name="faq" />
<p class="head">
Frequently Asked Questions
</p>
<a name="outofmemory">
<p class="smallhead">
java.lang.OutOfMemory Exception
</p>
<p>
By default a JVM places an upper limit on the amount of memory available
to the current process in order to prevent runaway processes gobbling
system resources and making the machine grind to a halt. When reading
or writing large spreadsheets, the JVM may require more memory than
has been allocated to the JVM by default - this normally manifests itself
as a java.lang.OutOfMemory exception.
<br /><br />
For command line processes, you can allocate more memory to the JVM
using the -Xms and -Xmx options eg. to allocate an
initial heap allocation of 10 mB, with 100 mB as the upper bound you can
use
<blockquote>
<a class="technical">
java -Xms10m -Xmx100m -classpath jxl.jar spreadsheet.xls
</a>
</blockquote>
In order to allocate more memory in this manner to servlets/JSPs, consult
the help documentation for the Web Application Server.
</p>
<a name="compiling" />
<p class="smallhead">
Compiling
</p>
<p>
The distribution of JExcelApi comes with a build.xml file. This may be
used by the build tool, ant, in order to build the software. If ant is
not already installed on your machine, it may obtained
<a href="http://jakarta.apache.org/ant">here</a>.
<br /><br />
To build API using ant simply change to the subdirectory called build,
from the command line, type
<blockquote>
<a class="technical">ant</a>
</blockquote>
This will detect any source files which have recent changes, compile
them and create the jar file in the parent directory.
<br /><br />
The build.xml specifies a number of targets. To totally rebuild the
whole application, including the javadoc documentation, then
obtain a command line prompt within the build directory and type
<blockquote>
<a class="technical">ant jxlall</a>
</blockquote>
As an alternative to using ant, JExcelApi may be built using the standard
java tools. From the command line in the build subdirectory
issue the following sequence of commands (modifying file separators and
classpath separators as required for the target operating system):
<blockquote>
<a class="technical">
javac -d out -classpath out:../src ../src/jxl/demo/*.java
<br />
jar cmf jxl.mf ../jxl.jar -C out common jxl
</a>
</blockquote>
</p>
<a name="uploading" />
<p class="smallhead">
Uploading spreadsheets via the browser
</p>
<p>
Below is some indicative code which may be used for uploading
spreadsheets from a client browser to servlet.
<br /><br />
In the HTML page which is displayed to the user requesting the
upload, declare a form of multipart form data:
<blockquote>
<a class="technical">
<form action="/test/upload" method="post" enctype="multipart/form-data"><br />
<input size="70" type="file" name="spreadsheet" /><br />
<br /><br /><br /><br />
<input type="submit" value="Upload File" /><br />
</form>
</a>
</blockquote>
The servlet which processes this code should access the input stream
directly. Because of the encoding method, it is not possible to use
the request.getParameter() methods.
<br /><br />
Accessing the input stream directly means that the HTTP information is
present. The first thing to do is strip off this redundant information
before passing the input stream directly to the API, thus:
<blockquote>
<a class="technical">
protected void doPost(HttpServletRequest request, HttpServletResponse response)<br />
throws IOException, ServletException<br />
{<br />
try<br />
{<br />
ServletInputStream is = request.getInputStream();<br />
byte[] junk = new byte[1024];<br />
int bytesRead = 0;<br />
<br />
// the first four lines are request junk<br />
bytesRead = is.readLine(junk, 0, junk.length);<br />
bytesRead = is.readLine(junk, 0, junk.length);<br />
bytesRead = is.readLine(junk, 0, junk.length);<br />
bytesRead = is.readLine(junk, 0, junk.length);<br />
<br />
Workbook workbook = Workbook.getWorkbook(is);<br />
<br />
<br />
// Do stuff with the workbook<br />
...<br />
}<br />
catch (JXLException e)<br />
{<br />
...<br />
}<br />
}<br />
</a>
</blockquote>
</p>
<a name="charts" />
<p class="smallhead">
Support for charts, macros and images
</p>
<p>
JExcelApi has limited support for charts: when copying a spreadsheet
containing a chart, the chart is written out to the generated
spreadsheet (as long as the sheet containing the chart contains
other data as well as the chart).
</p>
<p>
All macro and information is ignored. Consequently when copying
and writing out the macros that were in the original
will not be present in the generated version.
</p>
<p>
All image information is preserved when copying spreadsheets. When
adding an image to a spreadsheet only images in PNG format are
supported
</p>
<a name="dates" />
<p class="smallhead">
Date display
</p>
<p>
When displaying dates, the java.util package automatically adjusts for
the local timezone. This can cause problems when displaying dates
within an application, as the dates
look as if they are exactly one day previous to that which is stored
in the Excel spreadsheet, although this is not in fact the case.
</p>
<p>
Excel stores dates as a numerical value, and the conversion process
for transforming this into a java.util.Date consists of converting the
Excel number into a UTC value and then using the UTC number to create
the java Date. Say the number in Excel represents 20 August 2003, then
the UTC equivalent of this number is used to create a java.util.Date
object.
</p>
<p>
The problem occurs if you are operating in a timezone other than GMT.
As soon
as you try and perform any user IO on that java Date object (eg.
System.out.print(date)) the JVM will perform timezone adjustment
calculations. If you are located in EST zone
(which is GMT - 5 hours) java will subtract 5 hours
from the date - so the Date object instead of being 00:00 20/08/2003
becomes 19:00 19/08/2003. Because java recognizes you only want a date
and not a date time, it truncates the hours/minutes/seconds
and presents 19/08/2003 - so it appears that the day is one day
less than was stored in Excel, whereas it is really only a few hours
(the timezone offset) less. Needless to say, this is a very annoying
feature.
</p>
<p>
The easiest way to work around this (and the method used internally by
the getContents() method of a jxl.DateCell) is to force the timezone
of the date format as follows:
<blockquote>
<a class="technical">
TimeZone gmtZone = TimeZone.getTimeZone("GMT");<br />
SimpleDateFormat format = new SimpleDateFormat("dd MMM yyyy");<br />
format.setTimeZone(gmtZone);</br>
<br />
DateCell dateCell = .... <br />
String dateString = format.format(dateCell.getDate());<br />
</blockquote>
</p>
<a name="multiple workbooks" />
<p class="smallhead">
Cell Formats Across Multiple Workbooks
</p>
<p>
Sometimes a single process may generate multiple workbooks. When
doing this is it tempting to create the various cell formats once
(eg. as member data or as static constants) and apply them to cells
in both workbooks. This works fine for the first workbook, but for
subsequent workbooks this can cause unexpected cell formatting. The
reason for this is that when a format is first added to a workbook,
JExcelApi assigns an internal cross-reference number to that cell,
and all other cells which share this format simply store the
cross reference number. However, when you add the same cell
format to a different workbook, JExcelApi recognizes that the format
has been added to a workbook, and simply refers to the format by
the index number, rather than by initializing it properly.
When Excel then tries to read this workbook, it sees an index number,
but is unable to read the cell format (or reads a different one)
as the cell can be formatted in an unpredictable manner.
</p>
<p>
The long and the short of it is that if it is necessary to re-use formats
across multiple workbooks, then the WritableCellFormat objects must
be re-created and initialised along with the each Workbook instance, and
NOT re-used from a previous workbook.
</p>
<a name="cross sheet formulas" />
<p class="smallhead">
Cross Sheet Formulas
</p>
<p>
JExcelApi supports formulas across sheets. However, please make sure
all the sheets have been added to the workbook (even if they are blank)
before attempting to create cross sheet formulas. This is because if
you create a cross sheet formula referencing a sheet and then
subsequently and or remove sheets from the workbook, the sheet reference
in the formula when it was parsed won't necessarily reference the
correct sheet, and could even cause Excel to crash
</p>
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