import java.io.*;
import java.security.MessageDigest;

public class MD5Checksum {

   public static byte[] createChecksum(String filename) throws
       Exception{
     InputStream fis =  new FileInputStream(filename);

     byte[] buffer = new byte[1024];
     MessageDigest complete = MessageDigest.getInstance("MD5");
     int numRead;
     do {
      numRead = fis.read(buffer);
      if (numRead > 0) {
        complete.update(buffer, 0, numRead);
        }
      } while (numRead != -1);
     fis.close();
     return complete.digest();
   }

   // see this How-to for a faster way to convert a byte array to 
   // a HEX string 
   public static String getMD5Checksum(String filename) throws Exception {
     byte[] b = createChecksum(filename);
     String result = "";
     for (int i=0; i < b.length; i++) {
       result +=
          Integer.toString( ( b[i] & 0xff ) + 0x100, 16).substring( 1 );
      }
     return result;
   }

   public static void main(String args[]) {
     try {
       System.out.println(getMD5Checksum("apache-tomcat-5.5.17.exe"));
       // output :
       //  0bb2827c5eacf570b6064e24e0e6653b
       // ref :
       //  http://www.apache.org/dist/
       //          tomcat/tomcat-5/v5.5.17/bin
       //              /apache-tomcat-5.5.17.exe.MD5
       //  0bb2827c5eacf570b6064e24e0e6653b *apache-tomcat-5.5.17.exe
     }
     catch (Exception e) {
         e.printStackTrace();
     }
   }
}

import java.security.*;
import java.io.*;

public class Checksum {

  //
  // return 0 error
  //        1 ok (create)1
  //        1 same (check)
  //        2 different (check)
  //
  public static void main(String args[]) {
     if (args.length == 2) {
        if (args[0].equals("create")) {
           System.exit(new Checksum().create(args[1]));
        }
        else if (args[0].equals("check")) {
           System.exit(new Checksum().check(args[1]));
        }
     }
     else {
        System.out.println("Usage : java Checksum create [filename]\n"+
                           "        java Checksum check  [filename]");
     }
  }


  public int create(String filename){
     try {
        byte[] chk = createChecksum(filename);
        File f = new File(filename + ".chk");
        OutputStream os = new FileOutputStream(f);
        os.write(chk);
        os.close();
        return 1;
     }
     catch(Exception e) {
        e.printStackTrace();
        return 0;
     }
  }

  public int check(String filename){
     int rc = 0;
     try {
        byte[] chk1 = createChecksum(filename);
        byte[] chk2 = new byte[chk1.length];
        File f = new File(filename + ".chk");
        InputStream is = new FileInputStream(f);

        is.read(chk2);

        if (new String(chk2).equals(new String(chk1))) {
           System.out.println("Same!");
           rc = 1;
        }
        else {
          System.out.println("Different!");
          rc = 2;
          }
        is.close();
        return rc;
     }
     catch(Exception e) {
        e.printStackTrace();
        return rc;
     }
  }

  public byte[] createChecksum(String filename) throws Exception{
     InputStream fis =  new FileInputStream(filename);

     byte[] buffer = new byte[1024];
     MessageDigest complete = MessageDigest.getInstance("MD5");
     int numRead;
     do {
       numRead = fis.read(buffer);
       if (numRead > 0) {
         complete.update(buffer, 0, numRead);
       }
     } while (numRead != -1);
     fis.close();
     return complete.digest();
  }
}

import java.io.*;
import java.util.zip.CRC32;
import java.util.zip.Checksum;

public class ComputeCRC32 {
  public static long getChecksumValue(Checksum checksum, String fname) {
    try {
       BufferedInputStream is = new BufferedInputStream(
         new FileInputStream(fname));
       byte[] bytes = new byte[1024];
       int len = 0;

       while ((len = is.read(bytes)) >= 0) {
         checksum.update(bytes, 0, len);
       }
       is.close();
    }
    catch (IOException e) {
       e.printStackTrace();
    }
    return checksum.getValue();
    }

  public static void main(String[] args) {
    if (args.length != 1) {
      System.err.println("Usage: java ComputeCRC32 <file>");
      System.exit(1);
    }
    long cs = getChecksumValue(new CRC32(), args[0]);
    System.out.println("crc32 " + args[0] + " : " + cs);
  }
}