Easy to Learn Java: Programming Articles, Examples and Tips

Start with Java in a few days with Java Lessons or Lectures


Code Examples

Java Tools

More Java Tools!

Java Forum

All Java Tips


Submit News
Search the site here...
Search the JavaFAQ.nu
1000 Java Tips ebook

1000 Java Tips - Click here for the high resolution copy!1000 Java Tips - Click here for the high resolution copy!

Java Screensaver, take it here

Free "1000 Java Tips" eBook is here! It is huge collection of big and small Java programming articles and tips. Please take your copy here.

Take your copy of free "Java Technology Screensaver"!.

Follow-up to JDK 1.4 HashMap hashCode() mystery

JavaFAQ Home » Story by Dr. Kabutz Go to all tips in Story by Dr. Kabutz

Bookmark and Share

The Java Specialists' Newsletter [Issue 054b] - Follow-up to JDK 1.4 HashMap hashCode() mystery

Author: Dr. Heinz M. Kabutz

JDK version:

Category: Performance

You can subscribe from our home page: http://www.javaspecialists.co.za (which also hosts all previous issues, available free of charge Smile

This is just a quick follow-up to the newsletter sent this morning at 2:00am South African time. It seems I was not very careful with the source code and a few errors crept in. Please have a look at the archive for a corrected edition. After finally collapsing in bed at 2:15am, I was rudely reminded that I have a daughter of 11 months old. She kept up her antics until 6:00am, when I had to get up to carry on presenting my Java course. *sigh* - today was not a good day!

Joshua Bloch wrote to me after last night's newsletter, sending me some more information about the remainder performance mystery, and I feel I should pass the information on to you, my readers:

Joshua Bloch: By the way, I now know more about what's going on with mod/division. There is a collection of techniques for doing fast division by a constant. These techniques are covered in great detail in Chapter 10 of a marvelous new book with the unlikely title of "Hacker's Delight" by Henry Warren. It turns out that the old ("Classic") VM knew some of these tricks, but Hotspot, in releases up to 1.4, did not. While 1.4.1 can do some of this stuff, I suspect that later releases will do more.

I wrote a FairRemainderBenchmark that calculates the remainder with a variable, the way that the old HashMap would have done, and alas, the speed of the various JDKs is roughly the same:

import java.util.Random;
public class FairRemainderBenchmark implements Benchmark {
  private static final int ITERATIONS = 10 * 1000 * 1000;
  private int memory;
  public int doCalculation() {
    int val = 0;
    Random rand = new Random(0);      
    int bucket_size = (int)(rand.nextDouble() * 101) + 1;
    for (int i = 0; i < ITERATIONS; i++) {
      val = i % bucket_size;
    memory = val;
    return ITERATIONS;

The performance is now quite similar between the old and the new versions of the JVM:

JVM version:1.2
18867  18832  18484  19193  18832  18518  19193  18832  18484  18832
Average 18806 iterations per millisecond

JVM version:1.3.1_03
18867  18148  18832  18484  18484  18867  18832  18832  18148  18484
Average 18597 iterations per millisecond

JVM version:1.4.0
18832  19193  19230  19193  19193  19193  19193  19230  19193  19193
Average 19164 iterations per millisecond

JVM version:1.4.1-beta
17825  18148  18148  18148  17543  18484  18148  18148  17513  18518
Average 18062 iterations per millisecond


Copyright 2000-2004 Maximum Solutions, South Africa

Reprint Rights. Copyright subsists in all the material included in this email, but you may freely share the entire email with anyone you feel may be interested, and you may reprint excerpts both online and offline provided that you acknowledge the source as follows: This material from The Java(tm) Specialists' Newsletter by Maximum Solutions (South Africa). Please contact Maximum Solutions for more information.

Java and Sun are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and other countries. Maximum Solutions is independent of Sun Microsystems, Inc.

 Printer Friendly Page  Printer Friendly Page
 Send to a Friend  Send to a Friend

.. Bookmark and Share

Search here again if you need more info!
Custom Search

Home Code Examples Java Forum All Java Tips Books Submit News, Code... Search... Offshore Software Tech Doodling

RSS feed Java FAQ RSS feed Java FAQ News     

    RSS feed Java Forums RSS feed Java Forums

All logos and trademarks in this site are property of their respective owner. The comments are property of their posters, all the rest 1999-2006 by Java FAQs Daily Tips.

Interactive software released under GNU GPL, Code Credits, Privacy Policy