Exception in thread "main" java.lang.NullPointerException错误怎么解决

下面这段程序，编译没问题，但运行时候会报错
Exception in thread "main" java.lang.NullPointerException
at operation.GetFileInfo.getFileNum(GetFileInfo.java:20)
at operation.ExtractOperation.extraction(ExtractOperation.java:11)
at operation.ExtractOperation.main(ExtractOperation.java:30)
报错定位在下面红色的语句上，我知道这是说有空指针存在，可是我找不到。
麻烦大家帮我找找错误！！

此外，我的程序是从html源代码中匹配文字，然后输入到文件parameter.txt中，有一次运行成功了，建立了txt文件，却没内容，麻烦大家帮个忙！谢谢！

public class ExtractOperation {

   static String filePath = "./source/";

   public void extraction(String filePath) throws IOException{

	   GetFileInfo gfn = new GetFileInfo();

       filePath=filePath+gfn.getFileName(filePath)+"/";

       long num = gfn.getFileNum(filePath);         for(int i=1;i<=num;i++){

    	   char c[] = new char[3000]; //创建可容纳3000个字符的数组

    	   FileReader fr = new FileReader(filePath+String.valueOf(i)+"/"+"index"+String.valueOf(i)+".txt"); 

    	   int number = fr.read(c);  //将数据读入字符数组c内，并返回读取的字符数

    	   String str=new String(c,0,number);   //将字符串数组转换成字符串	

		   Extraction extra = new Extraction();

		   try{

		      extra.getAttri(str);

	       }

		   catch(IOException e){

		      e.printStackTrace();

	       }

		}

   }

   public static void main(String args[])

   {

	   ExtractOperation eo = new ExtractOperation();

		try {

			eo.extraction(filePath);		} catch (IOException e) {			

			e.printStackTrace();

		}

   }

}



public class GetFileInfo{

	   public String getFileName(String filePath)

	   {

		   String fileName=null;

		   int lastNum=0;

		   File file=new File(filePath);

		   String[] test=file.list();

		   lastNum=file.list().length;

		   fileName=test[lastNum-1];

		   return fileName;

	   }

	   public long getFileNum(String filePath)

	   {

		    long lastNum;

		    File file=new File(filePath);

		    lastNum=file.list().length;		    return lastNum;

	   }

}



public class  Extraction{

	

	public void getAttri(String str)throws IOException

		{

		File file = new File("./source/Parameter.txt");	

		BufferedWriter bw = new BufferedWriter(new FileWriter(file));

		String s1="(?<=<td class=(\\u0022)tdTitle(\\u0022)>)\\w+(?=</td>)";//匹配参数名称    	

		Pattern p1=Pattern.compile(s1);

		Matcher m1=p1.matcher(str);

		while(m1.find())

		{

			String s=m1.group();	

			bw.write(s);

		}

		bw.close();			

	}

}