Question

I want to “override” or “switch” some bytes (say the first 2048 bytes from the file) with another bytes array, in a file which size is 100MB.
I don’t want to read the whole file, because it takes a critic time for me to go over the whole file.

What I’ve tried so far:

FileOutputStream out = new FileOutputStream(file);

out.getChannel().write(buffer, position);

The new buffer array is in the same size.

I’m developing with java + eclipse Android application which need to to that.
I’ll be glad if someone could write me a piece of code that do the job.

Thanks in advance.

Answer 1

This overwrites the first 2048 bytes of the file with the contents of the data array.

final RandomAccessFile file = new RandomAccessFile(filename, "rw");
final FileChannel channel = file.getChannel();
final byte[] data = new byte[2048];          // lets say it's got the data you want
final ByteBuffer buff = ByteBuffer.wrap(data);

channel.position(0);                         // (we were already here, but as an example)
channel.write(buff);                         // writes the entire 2028 bytes from buff
channel.force(false);                        // (superfluous if you close() afterwards)
channel.close();                             // close the file descriptor