He just needs a digest, not a digitally signed digest, doesn't he?

See http://mindprod.com/jgloss/digest.html
http://mindprod.com/jgloss/md5.html
http://mindprod.com/jgloss/sha1.html
http://mindprod.com/jgloss/adler.html

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

/**
* Test MD5 digest computation
*
* @author Roedy Green
* @version 1.0
* @since 2004-06-07
*/
public class MD5
  {

  /**
   * test MD5 digest, requires Java 1.4+
   *
   * @param args not used
   */
  public static void main ( String[] args ) throws
UnsupportedEncodingException, NoSuchAlgorithmException
  {
     byte[] theTextToDigestAsBytes = "The quick brown fox jumped over
the lazy dog's back".getBytes( "8859_1" /* encoding */ );
     MessageDigest md = MessageDigest.getInstance( "MD5" );
     md.update( theTextToDigestAsBytes );
     byte[] digest = md.digest();
     System.out.println( digest.length );
     // 16 bytes, 128 bits long
  }
  }

.
I think Adler should suffice. It is quick and generates 32 bits, all
you need for a hashCode.

// generate an 32-bit Adler checksum/digest
import java.io.UnsupportedEncodingException;
import java.util.zip.Adler32;

public class Adler
  {
  /**
  * test Adler digest
  * @param args not used
  */
  public static void main ( String[] args ) throws
UnsupportedEncodingException
     {
     byte[] theTextToDigestAsBytes = "The quick brown fox jumped over
the lazy dog's back".getBytes( "8859_1" /* encoding */ );
     Adler32 digester = new Adler32();
     digester.update( theTextToDigestAsBytes );
     // getValue produces a long to conform to the Checksum
interface.
     // Actual result is 32 bits long not 64!
     int digest = (int) digester.getValue();
     System.out.println( Integer.toHexString( digest ) );
     }
  }