PrintWriter异常:字符串索引超出范围

我试图读取一个文件,然后将一些文本附加到文件中的某个位置(即@offset jab)。当我试图在偏移量jabjab处写入文件时,就会出现问题。怎么了?

文件内容:

Mi
<?xml Version="1.0"?>

_

File f = new File("data.dat");
    String brstring = null;
    String entrystring = null;
    try {
        BufferedReader br = new BufferedReader(new FileReader(f));
        String line;
        StringBuilder result = new StringBuilder();
        while ((line = br.readLine()) != null) {
        result.append(line+"\r\n");
        }
        br.close();
        System.out.print(result);
        int jabjab = result.indexOf("?>");
        System.out.println(jabjab);
        PrintWriter fo = new PrintWriter(f);
        fo.write("ok", jabjab, 2);
        fo.flush();
        fo.close();
    } catch (Exception ex) {
        System.out.print(ex.getMessage());
    }

控制台输出,包括错误:

Mi// output of the result string
<?xml Version="1.0"?>//output of the result string
23//output of jabjab
String index out of range: 25String index out of range: 25//output of exception