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Davanum Srinivas

// @(#)Sum.java	1.1 01/05/10
//
// Compute 16-bit checksums for a list of files, in the style of the BSD "sum"
// command.  Uses NIO mapped byte buffers for speed.

import java.io.*;
import java.nio.*;
import java.nio.channels.*;

public class Sum {
    // Compute a 16-bit checksum for all the remaining bytes
    // in the given byte buffer
    //
    private static int sum(ByteBuffer bb) {
	int sum = 0;
	while (bb.hasRemaining()) {
	    if ((sum & 1) != 0)
		sum = (sum >> 1) + 0x8000;
	    else
		sum >>= 1;
	    sum += bb.get() & 0xff;
	    sum &= 0xffff;
	}
	return sum;
    }

    // Compute and print a checksum for the given file
    //
    private static void sum(File f) throws IOException {

	// Open the file and then get a channel from the stream
	FileInputStream fis = new FileInputStream(f);
	FileChannel fc = fis.getChannel();

	// Get the file's size and then map it into memory
	int sz = (int)fc.size();
	MappedByteBuffer bb = fc.map(FileChannel.MAP_RO, 0, sz);

	// Compute and print the checksum
	int sum = sum(bb);
	int kb = (sz + 1023) / 1024;
	String s = Integer.toString(sum);
	System.out.println(s + "	" + kb + "	" + f);

	// Close the channel and the stream
	fc.close();
    }

    public static void main(String[] args) {
	if (args.length < 1) {
	    System.err.println("Usage: java Sum file...");
	    return;
	}
	for (int i = 0; i < args.length; i++) {
	    File f = new File(args[i]);
	    try {
		sum(f);
	    } catch (IOException x) {
		System.err.println(f + ": " + x);
	    }
	}
    }

}

More samples can be found at:
http://java.sun.com/j2se/1.4/docs/guide/nio/example/index.html