String str = null;
String strflg = null;
String str1 = "!";
String str2 = "@";

File fi = new File ("你的文档.TXT");
ReaderFile rf = new ReaderFile (fi);
BufferedReader br = new Reader();

//遍历文本，取出每一行。
while(str  = br.readline()!= null )
{
   //取出第一个字符。
   strflg = str.substring(0,1);
   if(strflg.equals(str1))
   {
      str = str.replace("!","@");
      System.ou.println(str);
   }
   else {System.ou.println(str);}

}

------解决方案--------------------

找找中文的jdk api文档看看吧！我初学！呵呵

------解决方案--------------------
简单方法不会,就会笨的
先把a.txt(原始数据)哪出来,改完后放入b.txt,然后再把b.txt写回去

Java code

import java.io.*;
public class Csdn1{
    public static void main(String[] args) {
        BufferedReader bd = null;
        BufferedWriter bw = null;
        try { bd  =  new BufferedReader(new FileReader("F:\\sushuai\\23\\a.txt"));
             bw = new BufferedWriter(new FileWriter("F:\\sushuai\\23\\b.txt"));                           
        
              String str1 = null;
              StringBuffer str2 = null;
              while((str1 = bd.readLine()) != null) {
                  // System.out.println(str1);
                      if (str1.length() == 0 ) {
                      } else if (str1.charAt(0) == '@'){
                      } else if (str1.charAt(0) == '!') {
                        str2 = new StringBuffer(str1);
                        str2.setCharAt(0,'@');
                        str1 =str2.substring(0);
                      }
              bw.write(str1,0,str1.length());
              bw.newLine();
         }
             bd.close();
             bw.close();
        }catch(IOException e) {
            e.printStackTrace();
         }
         try {
             bd = new BufferedReader(new FileReader("F:\\sushuai\\23\\b.txt"));
             bw = new BufferedWriter(new FileWriter("F:\\sushuai\\23\\a.txt"));
             String str3 = null;
             while((str3 = bd.readLine())!= null) {
                 bw.write(str3,0,str3.length());
                 bw.newLine();
             }
             bd.close();
             bw.close();
         } catch(IOException e1) {
             e1.printStackTrace();
         } 
    
    }
}

------解决方案--------------------

学习。

LZ 举的例子 太个例了