poi中总是显示org.apache.poi.hssf.usermodel不存在

import java.io.File;  
import java.io.FileInputStream;  
import java.io.FileNotFoundException;  
import java.io.IOException;  
import java.text.DecimalFormat;  
import java.text.SimpleDateFormat;  
import java.util.LinkedList;  
import java.util.List;  
import org.apache.poi.hssf.usermodel.HSSFCell;  
import org.apache.poi.hssf.usermodel.HSSFDateUtil;  
import org.apache.poi.hssf.usermodel.HSSFRow;  
import org.apache.poi.hssf.usermodel.HSSFSheet;  
import org.apache.poi.hssf.usermodel.HSSFWorkbook;  
import org.apache.poi.xssf.usermodel.XSSFCell;  
import org.apache.poi.xssf.usermodel.XSSFRow;  
import org.apache.poi.xssf.usermodel.XSSFSheet;  
import org.apache.poi.xssf.usermodel.XSSFWorkbook;  
public class Excel {
         public static void main(String argv[]){ 
         try{
             HSSFWorkbook wb=new HSSFWorkbook();//创建Excel对象
             HSSFSheet sheet=wb.createSheet("对象报表");//创建工作表单
             HSSFRow row = sheet.createRow(0);   
             HSSFCell cell=row.createCell(0);
             cell.setCellValue("单元格中的中文");
                         //输出文件
             FileOutputStream fileOut = new FileOutputStream("e:\\workbook.xls");
             wb.write(fileOut);
             fileOut.close();
             System.out.println("ok");
         }catch(Exception e){
                 e.printStackTrace();
         }}
}