jxl中对Excel表格操作出现的问题

package Excel;

import java.io.File;
import java.text.SimpleDateFormat;
import java.util.TimeZone;

import jxl.DateCell;
import jxl.Workbook;
import jxl.write.Label;
import jxl.write.WritableCellFormat;
import jxl.write.WritableFont;
import jxl.write.WritableSheet;
import jxl.write.WritableWorkbook;

public class CreateExcel {
public static void main(String args[]) {
try {
// 打开文件
WritableWorkbook book = Workbook.createWorkbook(new File(
" test.xls "));
// 生成名为“第一页”的工作表，参数0表示这是第一页
WritableSheet sheet = book.createSheet(" 第一页 ", 0);
TimeZone gmtZone = TimeZone.getTimeZone("GMT");
SimpleDateFormat format = new SimpleDateFormat("dd MMM yyyy");
format.setTimeZone(gmtZone);
DateCell dateCell = null;
String dateString = format.format(dateCell.getDate());
Label label5 = new Label(0, 6, dateString);
sheet.addCell(label5);
book.write();
book.close();

    } catch (Exception e) {
        System.out.println(e);
    }
}

}
出现 :java.lang.NullPointerException问题

dateCell你还没有赋值呢
[code="java"]
package examples;

import java.io.File;
import java.text.SimpleDateFormat;
import java.util.TimeZone;
import java.util.Date;

import jxl.DateCell;
import jxl.write.*;
import jxl.Workbook;
import jxl.write.Label;
import jxl.write.WritableCellFormat;
import jxl.write.WritableFont;
import jxl.write.WritableSheet;
import jxl.write.WritableWorkbook;

public class CreateExcel {
public static void main(String args[]) {
try {
// 打开文件
WritableWorkbook book = Workbook.createWorkbook(new File(
"/home/ss.xls "));
// 生成名为“第一页”的工作表，参数0表示这是第一页
WritableSheet sheet = book.createSheet(" 第一页 ", 0);
TimeZone gmtZone = TimeZone.getTimeZone("GMT");
SimpleDateFormat format = new SimpleDateFormat("dd MMM yyyy");
format.setTimeZone(gmtZone);

        DateTime dateCell = new DateTime(0, 5, new Date());
        sheet.addCell(dateCell);

        String dateString = format.format(dateCell.getDate());
        Label label5 = new Label(0, 6, dateString);
        sheet.addCell(label5);
        book.write();
        book.close();

    } catch (Exception e) {
        System.out.println(e);
    }
}

}
[/code]