About write read file

Question

I have log file, maximum 500kb, every 30 sec to the end of file I saveing new information and if filesize willbe bigger than 500kb I have to erase older string information in the begin of file and up hole file. How did this very fastly because if lost power file willbe bad. My code doesnt work.
public String ReadLn(InputStream InData) {
          int i = 0;
          int ch = 0;
          String line = "";
          try {
               while ((ch != -1) & (ch != 10)) {
                    ch = InData.read();
                    line += (char) ch;
          } catch (Exception e) {
               return null;
          if (line == "")
               return null;
          else
               return line;
/////Inserting new information
FileConnection fconn = (FileConnection) Connector.open(file,Connector.READ_WRITE);
               if (!fconn.exists()) {//file is present
                    return;
               OutputStream OutData = fconn.openOutputStream(fconn.fileSize());
               OutData.write((record+endline).getBytes());//record new information
               OutData.flush();
               OutData.close();
               OutData = fconn.openOutputStream();//seek to begin
               int ch = 0;
               InputStream InData = fconn.openInputStream();     
               String read_ln=ReadLn(InData);//read first line
               String data = "";
               byte buffer[] = new byte[1024];
               while ((ch=InData.read(buffer))!= -1) {
                    data = new String (buffer);
                    sendline(data);
                    OutData.write((data).getBytes());
               fconn.close();
               OutData.flush();
               OutData.close();
               InData.close();

Answer 1

Thanks for all. I will try to use RMS its more effectivity and simple to use and economy memory than txt file.