Hello Guys
Bellow code will help you to generate excel file in java.
Herebelow I have used apache poi jar file version: poi3.8.jar .
You can download jar file from given bellow link
http://poi.apache.org/download.html

Now Create ExcelExample.java and put bellow code

import java.io.File;
import java.io.FileOutputStream;
import java.util.HashMap;
import java.util.Map;
import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.CellStyle;
import org.apache.poi.ss.usermodel.Font;
import org.apache.poi.ss.usermodel.IndexedColors;
import org.apache.poi.ss.usermodel.PrintSetup;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;
import org.apache.poi.ss.util.CellRangeAddress;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

import com.sun.xml.internal.ws.client.RequestContext;

public class ExcelExample {


    public static void main(String ars[]) throws Exception {

        String[] titles = {"User Name", "Case Number", "Assigned on", "Last Modified" };

        Workbook wb;

        wb = new XSSFWorkbook();

        Map<String, CellStyle> styles = createStyles(wb);

        Sheet sheet = wb.createSheet("Report-Task By User");
        PrintSetup printSetup = sheet.getPrintSetup();
        printSetup.setLandscape(true);
        sheet.setFitToPage(true);
        sheet.setHorizontallyCenter(true);

        //title row
        Row titleRow = sheet.createRow(0);
        titleRow.setHeightInPoints(45);
        Cell titleCell = titleRow.createCell(0);
        titleCell.setCellValue("Report-Task By User");
        titleCell.setCellStyle(styles.get("title"));
        sheet.addMergedRegion(CellRangeAddress.valueOf("$A$1:$D$1"));

        //header row
        Row headerRow = sheet.createRow(1);
        headerRow.setHeightInPoints(40);
        Cell headerCell;
        for (int i = 0; i < titles.length; i++) 
        {
            headerCell = headerRow.createCell(i);
            headerCell.setCellValue(titles[i]);
            headerCell.setCellStyle(styles.get("header"));
                 //finally set column widths, the width is measured in units of 1/256th of a character width
            sheet.setColumnWidth(i, 30*256);
        }

       Cell dataCell;

       for(int i=2;i<10;i++)
       {

         Row dataRow = sheet.createRow(i);

         dataCell = dataRow.createCell(0);
         dataCell.setCellValue("Bhagwan"+i);
         dataCell.setCellStyle(styles.get("cell"));

         dataCell = dataRow.createCell(1);
         dataCell.setCellValue("123"+1);
         dataCell.setCellStyle(styles.get("cell"));

         dataCell = dataRow.createCell(2);
         dataCell.setCellValue(i+"/01/2015");
         dataCell.setCellStyle(styles.get("cell"));

         dataCell = dataRow.createCell(3);
         dataCell.setCellValue(i+"/01/2014");
         dataCell.setCellStyle(styles.get("cell"));

       }

        // Write the output to a file

        String file ="excelsheet.xls";


        if(wb instanceof XSSFWorkbook) file += "x";
        FileOutputStream out = new FileOutputStream(file);
        wb.write(out);
        out.close();
        System.out.print("success..........................");
        return file;
    }

    /**
     * Create a library of cell styles
     */
    private Map<String, CellStyle> createStyles(Workbook wb){
        Map<String, CellStyle> styles = new HashMap<String, CellStyle>();
        CellStyle style;
        Font titleFont = wb.createFont();
        titleFont.setFontHeightInPoints((short)18);
        titleFont.setBoldweight(Font.BOLDWEIGHT&#95;BOLD);
        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN&#95;CENTER);
        style.setVerticalAlignment(CellStyle.VERTICAL&#95;CENTER);
        style.setFont(titleFont);
        styles.put("title", style);

        Font monthFont = wb.createFont();
        monthFont.setFontHeightInPoints((short)11);
        monthFont.setColor(IndexedColors.WHITE.getIndex());
        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN&#95;CENTER);
        style.setVerticalAlignment(CellStyle.VERTICAL&#95;CENTER);
        style.setFillForegroundColor(IndexedColors.BLUE.getIndex());
        style.setFillPattern(CellStyle.SOLID&#95;FOREGROUND);
        style.setFont(monthFont);
        style.setWrapText(true);
        styles.put("header", style);

        style = wb.createCellStyle();
        style.setAlignment(CellStyle.ALIGN&#95;CENTER);
        style.setWrapText(true);
        style.setBorderRight(CellStyle.BORDER&#95;THIN);
        style.setRightBorderColor(IndexedColors.BLACK.getIndex());
        style.setBorderLeft(CellStyle.BORDER&#95;THIN);
        style.setLeftBorderColor(IndexedColors.BLACK.getIndex());
        style.setBorderTop(CellStyle.BORDER&#95;THIN);
        style.setTopBorderColor(IndexedColors.BLACK.getIndex());
        style.setBorderBottom(CellStyle.BORDER&#95;THIN);
        style.setBottomBorderColor(IndexedColors.BLACK.getIndex());
        styles.put("cell", style);


        return styles;
    }


}