db-jdo-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: Query objects and hashCode() + equals()
Date Mon, 16 Nov 2009 23:52:45 GMT
Hi Jörg,

On Nov 16, 2009, at 8:32 AM, Joerg von Frantzius wrote:

> Hi Craig,
>
> you are absolutely right about the Query object being mutable, and  
> this
> conflicting with its use as a key in a HashMap. But telling from the
> general contracts of Map, equals() and hashCode(), I don't think that
> there exists a requirement for Map keys to be immutable.

Right. What I said is that for use as an element in a Set or as a key  
in a Map, the instance should return the identical hashCode and behave  
identically with respect to equals. So while the instance itself does  
not have to be immutable, hashCode and equals should act on immutable  
properties of the instance.

As I understand how these collections work, if you do change how  
hashCode and equals behave after construction, the instance should be  
removed from the collection and added again after the change is done.  
This is pretty tricky in terms of synchronization and order of  
operation.

> There may also
> be other purposes for comparing Query objects using equals(), or Map
> implementations that don't require immutability of keys.
>
> I wouldn't ask if datanucleus wasn't already there halfway:  
> datanucleus'
> JDO query object delegates to org.datanucleus.store.query.Query, which
> already correctly implements equals(), hashCode() and toString().

So does the query object change hashCode when the filter changes?

> The
> only thing missing is the wrapper query object to also delegate these
> methods (I guess that was simply forgotten when that additional layer
> was introduced with JPOX implementing JPA).
>
> Just a small thing, I hope I'll find the time to fix it in datanucleus
> myself. When that has happened, it could still be worth putting in  
> the spec?

I don't yet understand the use case. Can you be specific as to what  
behavior you expect from a "proper implementation" of hashCode,  
equals, and toString, and how the implementation would be exploited by  
an application?

Regards,

Craig
>
> Regards,
> Jörg
>
> On 10/23/2009 06:57 PM, Craig L Russell wrote:
>> Hi Jörg,
>>
>> On Oct 16, 2009, at 9:50 AM, Joerg von Frantzius wrote:
>>
>>> Hi,
>>>
>>> it would be nice if the spec would mandate that implementations of
>>> javax.jdo.Query do correctly implement hashCode() and equals().
>>
>> As users execute mutator methods on queries, the string  
>> representation
>> would change. And for proper use in sets or map keys, hashCode and
>> equals should be  immutable after construction. So I don't see how we
>> can mandate that hashCode and equals rely in any internal state.
>>
>>> Alternatively, Query.toString() could be required to return the
>>> single-string representation of the query, or some dedicated  
>>> method was
>>> added to provide it.
>>>
>> Having the single-string version of a query is useful but adds
>> implementation complexity. While there is a requirement for an
>> implementation to parse a single string query into executable form,
>> there's no current requirement to create the single string form from
>> the internal form.
>>
>>> This would make it easier to e.g. implement some custom cache for  
>>> query
>>> results, or, more generally, it would make it easier to put Query
>>> objects as keys into Maps.
>>
>> It seems to me that if you want this functionality at the application
>> framework layer, then the framework can mandate that the single  
>> string
>> form be used and at the time the query is created, the single string
>> form be used as the key into a framework map.
>>
>> Regards,
>>
>> Craig
>>> ____________________________________________________________________
>>> artnology GmbH - Milastraße 4 - 10437 Berlin - Germany
>>> Geschäftsführer: Ekkehard Blome (CEO), Felix Kuschnick (CCO)
>>> Registergericht: Amtsgericht Berlin Charlottenburg HRB 76376
>>> UST-Id. DE 217652550
>>>
>>
>> Craig L Russell
>> Architect, Sun Java Enterprise System http://db.apache.org/jdo
>> 408 276-5638 mailto:Craig.Russell@sun.com
>> P.S. A good JDO? O, Gasp!
>>
>
>
> -- 
> ____________________________________________________________________
> artnology GmbH - Milastraße 4 - 10437 Berlin - Germany
> Geschäftsführer: Ekkehard Blome (CEO), Felix Kuschnick (CCO)
> Registergericht: Amtsgericht Berlin Charlottenburg HRB 76376
> UST-Id. DE 217652550
>

Craig L Russell
Architect, Sun Java Enterprise System http://db.apache.org/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


Mime
View raw message