Re: Hash table performance

From:
"John B. Matthews" <nospam@nospam.invalid>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 21 Nov 2009 12:54:53 -0500
Message-ID:
<nospam-01E554.12545321112009@news.aioe.org>
In article <T9GdnWzZ_sPfvJXWnZ2dnUVZ7vadnZ2d@brightview.co.uk>,
 Jon Harrop <jon@ffconsultancy.com> wrote:

I'm having trouble getting Java's hash tables to run as fast as .NET's.
Specifically, the following program is 32x slower than the equivalent
on .NET:

  import java.util.Hashtable;
  
  public class Hashtbl {
    public static void main(String args[]){
      Hashtable hashtable = new Hashtable();
  
      for(int i=1; i<=10000000; ++i) {
        double x = i;
        hashtable.put(x, 1.0 / x);
      }
  
      System.out.println("hashtable(100.0) = " + hashtable.get(100.0));
    }
  }

My guess is that this is because the JVM is boxing every floating point
number individually in the hash table due to type erasure whereas .NET
creates a specialized data structure specifically for a float->float hash
table with the floats unboxed. Consequently, the JVM is doing enormously
numbers of allocations whereas .NET is not.

Is that correct?


In addition to using HashMap, you might also specify an appropriate
initialCapacity and loadFactor. You may be seeing a lot of rehash
operations.

<http://java.sun.com/javase/6/docs/api/java/util/HashMap.html>

--
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>

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