[freenet-dev] [freenet-cvs] r18969 - trunk/freenet/src/freenet/clients/http/bookmark

[Matthew Toseland]

On Saturday 05 April 2008 04:22, Florent Daignière wrote:
> * Matthew Toseland <toad at amphibian.dyndns.org> [2008-04-04 19:13:51]:
> 
> > On Friday 04 April 2008 06:27, nextgens at freenetproject.org wrote:
> > > Author: nextgens
> > > Date: 2008-04-04 05:27:02 +0000 (Fri, 04 Apr 2008)
> > > New Revision: 18969
> > > 
> > > Modified:
> > >    trunk/freenet/src/freenet/clients/http/bookmark/BookmarkItem.java
> > > Log:
> > > implement BookmarkItem.hashCode()
> > 
> > Again, two points:
> > - This will change, and therefore break containing HashSet's etc, if the 
parts 
> > cease to be null. Are they all essential? If they are, are they final, and 
> > can they be null?
> 
> We must do the same checks as for equals()... If it breaks the code somehow 
we should fix it.

Of course, but I'm not sure we are doing this here:
- It is *NOT* necessary for hashCode to always return different values for 
different objects. We can ignore elements with impunity.
- equals() is true if two BookmarkItem's have the same key and different 
editions. So your code breaks equals/hashCode consistency.
- key and name cannot in any case be null.

I've fixed the above.

Also you haven't explained the mathematical voodoo... hashtables will 
generally depend on the first few bits, which will in this case depend on the 
description's hash followed by the alerts hash. Wouldn't it be better to 
simply XOR all the parts together?
> 
> "This is what the JDK 1.4 API documentation says about the hashCode method 
of Object class-
> 
> Returns a hash code value for the object. This method is supported for the 
benefit of hashtables such as those provided by java.util.Hashtable.
> 
>     * The general contract of hashCode is: Whenever it is invoked on the 
same object more than once during an execution of a Java application, the 
hashCode method must consistently return the same integer, provided no 
information used in equals comparisons on the object is modified. This 
integer need not remain consistent from one execution of an application to 
another execution of the same application.
>     * If two objects are equal according to the equals(Object) method, then 
calling the hashCode method on each of the two objects must produce the same 
integer result.
>     * It is not required that if two objects are unequal according to the 
equals(java.lang.Object) method, then calling the hashCode method on each of 
the two objects must produce distinct integer results. However, the 
programmer should be aware that producing distinct integer results for 
unequal objects may improve the performance of hashtables.
> 
> As much as is reasonably practical, the hashCode method defined by class 
Object does return distinct integers for distinct objects. (This is typically 
implemented by converting the internal address of the object into an integer, 
but this implementation technique is not required by the JavaTM programming 
language.) "
> 
> http://www.geocities.com/technofundo/tech/java/equalhash.html
> 
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