New to Java

Hi,
I am trying to write a simple FTP-eque client and server (an assignment) and am confused about how to read and write from DataInputStreams and DataOutputStreams. So to try to understand these better I wrote a little program to simple read a file with a DataInputStream and then write it to a new file with a DataOutputStream. Since the file may not be text based I thought it best to use the write() and read() methods rather than something like writeBytes(). I figure that the read-write should go like this:
- read a given number of bytes from the old file with DataInputStream, store in a byte array
- write the bytes in that array to a new file with DataOutputStream
- continue until the end of the old file

I would really appreciate some insight on how to 'correctly' handle chaining these streams, and/or an example of how to do so.

My code:

import java.io.*;
import java.net.*;

public class FileIn {

	public static void main (String args[]) throws Exception {
	
		String file = "/home/craig/oldfile.txt";
		String newfile = "/home/craig/newfile.txt";
		File f1 = new File(file);
		File f2 = new File(newfile);

		DataInputStream din = null;
		DataOutputStream dout = null;
		din = new DataInputStream( new FileInputStream(f1) );
		dout = new DataOutputStream( new FileOutputStream(f2) );

		int len = 1024;
		byte[] buf = new byte[len];
		byte[] b = null;
		
		while ( (b = din.read(buf)) != null ) {
			dout.write(b); 
		}
		dout.flush();
		dout.close();
	}
}