出现以下错误,请那位帮忙解决. Exception in thread "main" java.lang.NullPointerException

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出现以下错误,请那位帮忙解决. Exception in thread "main" java.lang.NullPointerException 15

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

/**
* 考勤管理类
*/
class AttendanceManager {

/**
* CSV中的文件条目
*/
class Item {
/** 计算机名 */
private String computerNum;

/** 登录状态 */
private String loginStatus;

/** 签到状态 */
private String signatureStatus;

/** 学生名 */
private String studentName;

/**
   * 根据一个字符串来初始化一个Item对象
   *
   * @param string
   */
public Item(String string) {
   initByString(string);
}

public String getComputerNum() {
   return computerNum;
}

public String getLoginStatus() {
   return loginStatus;
}

public String getSignatureStatus() {
   return signatureStatus;
}

public String getStudentName() {
   return studentName;
}

/**
   * 通过字符串初始化对象
   *
   * @param string
   */
private void initByString(String string) {
   String args[] = string.split(",");

   String info = null;
   for (int i = 0; i < args.length; i++) {
    info = args[i];
    switch (i) {
    case 0:
     setComputerNum(info);
     break;
    case 1:
     setStudentName(info);
     break;
    case 2:
     setLoginStatus(info);
     break;

    case 3:
     setSignatureStatus(info);
     break;

    default:
     break;
    }
   }
}

public void setComputerNum(String computerNum) {
   this.computerNum = computerNum;
}

public void setLoginStatus(String loginStatus) {
   this.loginStatus = loginStatus;
}

public void setSignatureStatus(String signatureStatus) {
   this.signatureStatus = signatureStatus;
}

public void setStudentName(String studentName) {
   this.studentName = studentName;
}
}

/**
* 结果对象
*/
class Result {

/**
   * 学生信息类
   */
class Student {
   /** 出勤次数 */
   private Integer signatureCount;

   /** 学生姓名 */
   private String studentName;

   public Student(String studentName, Integer signatureCount) {
    this.studentName = studentName;
    this.signatureCount = signatureCount;
   }

   public int compareTo(Student student) {
    if (student.getSignatureCount() == signatureCount) {
     return 0;
    } else if (student.getSignatureCount() > signatureCount) {
     return 1;
    } else if (student.getSignatureCount() < signatureCount) {
     return -1;
    }
    return 0;
   }

   public Integer getSignatureCount() {
    return signatureCount;
   }

   public String getStudentName() {
    return studentName;
   }

   public void setSignatureCount(Integer signatureCount) {
    this.signatureCount = signatureCount;
   }

   public void setStudentName(String studentName) {
    this.studentName = studentName;
   }
}

private Map<String, Integer> resultMap = new HashMap<String, Integer>();

/** 考勤总天数(文件个数) */
private int totalDay;

/**
   * 增加
   *
   * @param item
   */
public void add(Item item) {
   if (item.getStudentName().startsWith("N"))
    return;

   if (resultMap.containsKey(item.getStudentName())) {
    resultMap.put(item.getStudentName(), resultMap.get(item
      .getStudentName()) + 1);
   } else {
    resultMap.put(item.getStudentName(), new Integer(1));
   }
}

public int getTotalDay() {
   return totalDay;
}

public void setTotalDay(int totalDay) {
   this.totalDay = totalDay;
}

@Override
public String toString() {
   List<Student> students = new LinkedList<Student>();
   for (Map.Entry<String, Integer> entry : resultMap.entrySet()) {
    students.add(new Student(entry.getKey(), entry.getValue()));
   }

   // 排序
   Collections.sort(students, new Comparator<Student>() {
    @Override
    public int compare(Student o1, Student o2) {
     return o1.compareTo(o2);
    }
   });

   StringBuffer sb = new StringBuffer();
   sb.append("学生姓名,\t出勤次数,\t 缺勤次数,\t出勤分数\n");
   for (Student student : students) {
    sb.append(student.getStudentName());
    sb.append(",\t");
    sb.append(student.getSignatureCount());
    sb.append(",\t");
    sb.append(totalDay - student.getSignatureCount());
    sb.append(",\t");

    Float scoreFloat = (100F - (totalDay - student
      .getSignatureCount())
      * (100F / totalDay));
    int length = Math.min(4, scoreFloat.toString().length());
    sb.append(scoreFloat.toString().substring(0, length));
    sb.append("%\n");
   }
   String returnString = sb.toString();
   return returnString;
}
}

/** 考勤文件夹路径 */
private String filesDir;

/** 标识是否执行成功 */
private boolean isWorkSuccess;

/** 结果对象 */
private Result result = new Result();

/** 结果字符串 */

public String getFilesDir() {
return filesDir;
}

/**
* 获取结果
*/
public Result getResult() {
if (!isWorkSuccess) {
   throw new RuntimeException("请先执行work函数,或work时出错！");
}

return result;
}

/**
* 判断是否为考勤文件中具体的学生考勤信息
*
* @param readLine
* @return
*/
private boolean isData(String readLine) {
if (null == readLine || 0 == readLine.length())
   return false;

// 如果字符串以 "N" 开头，则认为为具体数据
if (readLine.startsWith("N")) {
   return true;
}
return false;
}

/**
* 分析单个考勤文件
*
* @param file
*            考勤文件
*/
private void parse(File file) {
try {
   // 一行一行地读取
   BufferedReader reader = new BufferedReader(new InputStreamReader(
     new FileInputStream(file)));

   String lineData = null;
   while (true) {
    lineData = reader.readLine();

    if (null == lineData)
     break;

    // 如果不是学生数据,则换成
    if (!isData(lineData))
     continue;

    Item item = new Item(lineData);
    result.add(item);
   }
} catch (Exception e) {
   System.out.println("解析文件" + file.getName() + "时发生错误！");
   e.printStackTrace();
}
}

public void setFileDir(String fileDir) {
this.filesDir = fileDir;
}

/**
* 设置考勤文件所在路径
*
* @param filesDir
*            文件路径
*/
public void setFilesDir(String filesDir) {
this.filesDir = filesDir;
}

/**
* 解析
*/
public void work() {
String path = getFilesDir();
    System.out.println(path);
File filesPath = new File(path);
File[] files = filesPath.listFiles();
result.setTotalDay(files.length);

for (File file : files) {
   parse(file);
}
isWorkSuccess = true;
}

/**
* 写入到文件
*
* @param filePath
*            文件路径
* @throws FileNotFoundException
*/
public void writeToFile(String filePath) throws FileNotFoundException {
PrintWriter writer = new PrintWriter(new File(filePath));
writer.println(result.toString());
writer.close();
}

}

public class Starter {
/**
* 学生考勤系统的设计和实现具有如下功能：
* 1、根据教师提供的excel考勤文件夹，统计学生出勤次数，缺勤次数，出勤分数（设考勤次数为N次，满勤为100分，缺勤1次扣总分的1/N）,并按分数排序。
* 2、统计结果在屏幕上显示出来，并保存到文本文件.txt之中。
*
*
* 分析:文件格式为CSV, 内容格式为: 计算机名,学生名,登录状态,签到状态
*
* @throws IOException
*/
public static void main(String[] args) {
AttendanceManager attendanceManager = new AttendanceManager();

// 设置考勤文件所在路径
attendanceManager
    .setFilesDir("C:\\07级2班点名2009.csv");

// 分析
attendanceManager.work();

// 获取结果
System.out.println(attendanceManager.getResult());

// 写入到文件
try {
   attendanceManager.writeToFile("c:/c.txt");
} catch (FileNotFoundException e) {
   System.err.println("写入文件错误！");
   e.printStackTrace();
}
}
}

出现以下错误,请那位帮忙解决.
Exception in thread "main" java.lang.NullPointerException
at AttendanceManager.work(Starter.java:324)
at Starter.main(Starter.java:364)

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