import org.apache.poi.poifs.filesystem.POIFSFileSystem;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.hssf.usermodel.HSSFSheet;
import org.apache.poi.hssf.usermodel.HSSFRow;
import org.apache.poi.hssf.usermodel.HSSFCell;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class setFormulaTest
{
    public static void main(String[] args)
        throws IOException
    {
        POIFSFileSystem fs      =
                new POIFSFileSystem(new FileInputStream("workbook.xls"));
        HSSFWorkbook wb = new HSSFWorkbook(fs);
        HSSFSheet sheet = wb.getSheetAt(0);
        HSSFRow row = sheet.getRow(0);
        HSSFCell cell = row.getCell((short)0);
        
	cell.setCellFormula("B1");
	
        FileOutputStream fileOut = new FileOutputStream("workbook.xls");
        wb.write(fileOut);
        fileOut.close();

    }
}