Reading and writing to a text file at particular position

I have a text file and I want to read and write a particular string at a specific position in the file.Like for example after " class:" word I want to write "AAA" in the next line.How can I do it using Java.
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rodnessConnect With a Mentor Commented:
The problem with doing this is that unless your file is guaranteed small enough to fit into available memory, you cannot assume that this will work.  THIS IS NOT A GOOD GENERAL SOLUTION.

With that said...

% more filename.txt
Blah blah blah...
blah blah blah.

% more

class Foo {
        public static void main(String[] args) throws Exception {
                // find the file length, create the appropriate byte array
                File file = new File( "filename.txt" );
                byte[] bytes = new byte[ (int) file.length() ];
                // read the file into the byte array.
                FileInputStream fis = new FileInputStream( file );
       bytes );
                // put the file contents into a StringBuilder
                StringBuilder sb = new StringBuilder( new String( bytes ) );
                // find the location of the line to insert after, plus
                // the length of the line itself.
                String token = "Class:";
                int offset = sb.indexOf( token ) + token.length();
                // insert what you want to insert at this point:
                sb.insert( offset, "AAA" );
                // write it out to the same file.
                FileOutputStream fos = new FileOutputStream( file );
                fos.write( sb.toString().getBytes() );
% javac
% java Foo
% more filename.txt
Blah blah blah...
blah blah blah.

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Note I have not bothered with error checking.  This is just an example.

Judging by discussions below it looks like there is no good way to do it,
you want probably re-write the whole stuff  to a new file insert a line at some point
and then  delete the old one and rename the new one to the old name
If you want to replace the stuff (not insert) you probably can manage to do something with RandomAccessFile,
though that should be done with great care. I'd rather re-write
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This example shows a lot of manipulations with RandomAccessFiles:
vgsrikanthAuthor Commented:
If you dont mind, can you please give me a piece of code on how to go about
vgsrikanthAuthor Commented:
I am reading the file and when I come across "class:" line I have to append a line of text like "AAA" in the next line of "class:".I dont want to write to a new file but want to read a existing file and make modifications to it.Can you please give me the code in Java.Thank you in advance.
I don't think you can append some stuff to the existing file in the middle of it - you'll have to re-write it.
File does not have empty spaces in there, so if you add some bytes to a line,
then you'll overwrite the following bytes. At most, you can replace some bytes, but you cannot add in the middle
of the file. Well, let's say, I never did it, and don't know how to do it.
You cannot do this in Java, or any programming language.

A text file is simply a sequence of character bytes.  You CANNOT INSERT new characters at a specific point in the file WITHOUT OVERWRITING data after that point.

Even manipulation with RandomAccessFile is not going to do what you want, because it is only capable of writing data at a particular location of the file, but this DOES NOT INSERT data, it only overwrites the file data.

The only workable solution is to read in the file and write it back out again, modifying (changing/inserting/deleting) what you want to modify at the appropriate points in the file.
vgsrikanthAuthor Commented:
Can someone give me the code in Java on how to read the file, store it in say byte array and checking the byte array for the string "class:" adding the new text "AAA" in the next line of "class:" and writing on to a new file.Thank you for your help.
What would be still normal and sufficiently general solution is to do obvious
thing - open your file, open another empty file, read your file line by line,
write to the new file line by line
once you see "class" add "AAA" and then write this modified  line or additional line if you need
and then continue re-writing up to the end.
There is nothing better than the good old straightforward way...
vgsrikanthAuthor Commented:
Thank you all for sharing your thoughts and rodness for your code.It is working.
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