[liftthis]

I need some help I am trying to parse a file for the last 40 bytes of data.

My file could be x number of bytes long. I only want the last 40 bytes of data to save as string, and then I want to replace the file getting ride of the 40 bytes of data.

public String removeHdr(File file1, File replaced) 
{
    	long sizeOfFile =0;

    	sizeOfFile = file1.length();

	System.out.println("file size " + sizeOfFile );

	return null;
    	
 }

[CasiOo]

Save the read chars/bytes in a ByteBuffer(havent tried using this before) so you can rewrite the file without the last 40 bytes
My example below does only get the last 40 bytes, it does not save the leading bytes
Instead of skipping (in my example) you should read the bytes Just keep track of how many you need to read and it should be no problem

long offset = file.lengt() - 40; //We only want to read the last 40 bytes

if (offset > 0) {
	FileInputStream reader = new FileInputStream("myfile.txt");
	
	//Skip until there are just 40 bytes left in the stream
	long remainingSkips = offset; //How many bytes we still have to skip
	while (remainingSkips != 0) 
		remainingSkips -= reader.skip(remainingSkips);
	
	//We should now have 40 bytes left
	byte[] buffer = new byte[40];
	reader.read(buffer);
	return new String(buffer);
}

[farrell2k]

I wouldn't use a loop.

public static void main(String[] args) {
        FileInputStream fis = null;
        File file = new File("files/file.txt");
        byte[] b = new byte[40];

        try {
            fis = new FileInputStream(file);
            
            long skipSize = file.length() - (file.length() - 40); 
            
            fis.skip(skipSize);
            
            fis.read(B)/>;
            
            String s = new String(B)/>;
            
            System.out.println(s);
            
        } 
        catch (IOException ex) { 
            Logger.getLogger(FileTest.class.getName()).log(Level.SEVERE, null, ex);
        }
   
    }//end main

This is a blocking operation that should be done off your main Thread.

[CasiOo]

farrell2k, on 09 October 2012 - 06:11 AM, said:

I wouldn't use a loop.

public static void main(String[] args) {
        FileInputStream fis = null;
        File file = new File("files/file.txt");
        byte[] b = new byte[40];

        try {
            fis = new FileInputStream(file);
            
            long skipSize = file.length() - (file.length() - 40); 
            
            fis.skip(skipSize);
            
            fis.read(B)/>;
            
            String s = new String(B)/>;
            
            System.out.println(s);
            
        } 
        catch (IOException ex) { 
            Logger.getLogger(FileTest.class.getName()).log(Level.SEVERE, null, ex);
        }
   
    }//end main

This is a blocking operation that should be done off your main Thread.

You need the loop if you read how skip works

Skip does not guarantee to skip the bytes you tell it to, instead it returns how many bytes it was able to skip. That is why you need the loop
I agree that it should work like in your example, but sadly it does not and it has caused problems for many programmers

[farrell2k]

Good catch

[g00se]

A RandomAccessFile is perfect for this job:

    public static String tailAndTruncate(String filePath, int tailSize) {
        /**
         * Get the last 'tailSize' bytes from a file as a String
         * and remove that from the original file
         */
        String result = null;
        RandomAccessFile raf = null;

        try {
            raf = new RandomAccessFile(filePath, "rw");
            raf.seek(raf.length() - tailSize);

            byte[] tailBytes = new byte[tailSize];
            raf.read(tailBytes);
            result = new String(tailBytes);
            raf.setLength(raf.length() - tailSize);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                raf.close();
            } catch (IOException e) { /* ignore */
            }
        }

        return result;
    }

[pbl]

using file.length() you can skip the number of bytes - 40
or open the file as a RandomFile and reads only strarting from the end - 40

Sorry g00se I missed your post