Question

EDIT
This is my file reader, can I make this read it from bottom to up seeing how difficult it is to make it write from bottom to up.

        BufferedReader mainChat = new BufferedReader(new FileReader("./messages/messages.txt"));
    String str;
    while ((str = mainChat.readLine()) != null) 
    {
        System.out.println(str);
    }
    mainChat.close();

OR (old question)

How can I make it put the next String at the beginning of the file and then insert an new line(to shift the other lines down)?

FileWriter chatBuffer = new FileWriter("./messages/messages.txt",true);
BufferedWriter mainChat = new BufferedWriter(chatBuffer);
mainChat.write(message);
mainChat.newLine();
mainChat.flush();
mainChat.close();

Answer 1

Someone could correct me, but I’m pretty sure in most operating systems, there is no option but to read the whole file in, then write it back again.

I suppose the main reason is that, in most modern OSs, all files on the disc start at the beginning of a boundary. The problem is, you cannot tell the file allocation table that your file starts earlier than that point.

Therefore, all the later bytes in the file have to be rewritten. I don’t know of any OS routines that do this in one step.

So, I would use a BufferedReader to store whole file into a Vector or StringBuffer, then write it all back with the prepended string first.

—

Edit

A way that would save memory for larger files, reading @Saury’s randomaccessfile suggestion, would be:

file has N bytes to start with
we want to add on "hello world"
open the file for append
append 11 spaces
i=N
loop {
   go back to byte i
   read a byte 
   move to byte i+11
   write that byte back
   i--
} until i==0
then move to byte 0
write "hello world" 

voila