注意每个方法结束都要关闭workbook；

还有getIdbyname（）方法中字符串flag与name的比较，一定要用equals()方法！！！；

剩下的不多解释，注释都在代码中：

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;

import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.CellType;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.ss.usermodel.Workbook;

public class DnsTest {
    public static void main(String[] args) throws IOException{
        getIdbyname("株洲");
        getNamebyid(6) ;  
    }
    public static void getIdbyname(String name)throws IOException  {
        //通过name获得id    
        String filePath="D://dns.xls";
         InputStream input = new FileInputStream(filePath);
            Workbook wb = null;
            wb = new HSSFWorkbook(input);
            //得到一个工作表对象;
            Sheet sheet = wb.getSheetAt(0);
            int rsRows = sheet.getLastRowNum();// 获取sheet表中的总行数            
             // 遍历行
        for (int i=0;i<=rsRows;i++) {
            System.out.println("遍历行数"+i);
            Row row = sheet.getRow(i);
            int id=0;
            String flag=null;
            //遍历行单元格，已知有两列;第一列int型id,第二列String型name
                Cell cell1 = row.getCell(0);
                Cell cell2 = row.getCell(1);
                if(cell1==null||cell1.equals(null)||cell1.getCellType()==CellType.BLANK){
                    System.out.println("id为空");
                    break;
                }else {
                    //数值型
                    id=(int) cell1.getNumericCellValue();
                    
                };
                if(cell2==null||cell2.equals(null)||cell2.getCellType()==CellType.BLANK){
                    System.out.println("name为空");
                     break;
                }else {
                    //字符串型
                    flag= cell2.getStringCellValue();
                };    
                String a=new String(flag); 
                String b=new String(name);
                if(a.equals(b)){
                    System.out.println(id);
                };
        }                    
        wb.close();//记得关闭
    }

    public static void getNamebyid(int id) throws IOException {
        //通过id获得name

                String filePath="D://dns.xls";
                InputStream input = new FileInputStream(filePath);
                Workbook wb = null;
                wb = new HSSFWorkbook(input);
                //得到一个工作表对象;
                Sheet sheet = wb.getSheetAt(0);
                int rsRows = sheet.getLastRowNum();// 获取sheet表中的总行数            
                 // 遍历行
                for (int i=0;i<=rsRows;i++) {
                    int flag=0;
                    String name=null;
                    Row row = sheet.getRow(i);
                    //遍历行单元格
                    Cell cell1= row.getCell(0);
                    Cell cell2 = row.getCell(1);
                    if(cell1==null||cell1.equals(null)||cell1.getCellType()==CellType.BLANK){
                        break;
                    }else {
                        //数值型
                        flag=(int) cell1.getNumericCellValue();
                        
                    }
                    if(cell2==null||cell2.equals(null)||cell2.getCellType()==CellType.BLANK){
                         break;
                    }else {
                        //字符串型
                        name= cell2.getStringCellValue();
                    }
                    if(flag==id){
                        System.out.println(name);
                    }
                }                    
                wb.close();//记得关闭    
    }
}