On Saturday 05 April 2008 04:22, Florent Daigni?re 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 > -------------- next part -------------- A non-text attachment was scrubbed... Name: not available Type: application/pgp-signature Size: 189 bytes Desc: not available URL: <https://emu.freenetproject.org/pipermail/devl/attachments/20080405/26a72d01/attachment.pgp>
