关于 java String字符串截取问题

/*
 需求：当前有一张关键词表和一个新闻文本
 要把在文本中所包含的词表中的关键词标识出来，（在这里我用的是前后加标识符#和**）
 然后输出到另一个文本
 */
 import java.io.*;


 public class adverbSignd {
 static File txtFile;// 将所有路径定义为全局变量
 static File advFile;
 static File targetFile;
 static String beginSign = "#";
 static String endSign = "**";


 adverbSignd(File txtFile, File adverbFile, File targetFile)
 throws Exception {
 this.txtFile = txtFile;
 this.advFile = adverbFile;
 this.targetFile = targetFile;
 get();
 }


 public void get() throws Exception {
 FileInputStream fisTxt = new FileInputStream(txtFile);
 BufferedReader bufAdv = new BufferedReader(new FileReader(advFile));
 BufferedWriter bwTar = new BufferedWriter(new OutputStreamWriter(
 new FileOutputStream(targetFile), "utf-8"));


 // 将原文件读取至数组和txtStr中
 int txtLenth = fisTxt.available();
 byte[] txt = new byte[txtLenth];
 fisTxt.read(txt);
 String txtStr = new String(txt);


 // 将原字符串每个字符和词汇表每个词比较


 for (int i = 0; i < txtStr.length() - 1;) {
 String departStr = null;
 String line = null;
 boolean bool = false;//判断关键字表中是否有此关键字或词
 while ((line = bufAdv.readLine()) != null) {
 departStr = txtStr.substring(i, i + line.length());//读取与关键字等长的子串
 if (departStr.equals(line)) {
 bwTar.append(beginSign + line + endSign);
 bool = true;
 i = i + line.length();// 若找到此关键词，那么源字符串的角标向后移动此关键字的长度个位置

 }


 }
 if (bool == false) {// 如果在整个词表都没有匹配到，那么将这一个字符输出，角标向后移一位。
 bwTar.append(txtStr.subSequence(i, i + 1));
 i = i + 1;
 }
 bwTar.flush();
 //System.out.println(departStr);
 }


 fisTxt.close();
 bufAdv.close();
 bwTar.close();
 }


 public static void main(String[] args) throws Exception {
 txtFile = new File("C:\\Users\\Administrator\\Desktop\\新闻.txt");
 advFile = new File("C:\\Users\\Administrator\\Desktop\\词表.txt");
 targetFile = new File("C:\\Users\\Administrator\\Desktop\\结果.txt");
 new adverbSignd(txtFile, advFile, targetFile);


 }
 }

//结果原文本原样输出到目标文本，