[OpenJDK 2D-Dev] Review Request for JDK-8153943 : In PixelInterLeavedSampleModel and BandedSampleModel we dont need hashCode() method

Jim Graham james.graham at oracle.com
Mon Jun 27 23:05:50 UTC 2016 

This is odd that two completely different classes have identical "equals()" methods.  After looking into it in more 
detail it looks as if ComponentSM is implemented as a general case that can encompass either PixelInterleaved or Banded 
pixel layouts - which means the subclasses are mostly just cosmetic (offering the constructors that make most sense if 
the pixels are laid out in the different ways) and only Banded offers a different implementation of getDataElements 
which is only different from the super implementation by virtue of eliminating a "multiply by a number which we know to 
be 1".

There are also some restrictions in the constructors that enforce limits on the various values that CSM allows in its 
general implementation, so it isn't possible to use the PixelInterleaved constructor to create an arbitrarily-valued CSM 
nor to use the Banded constructors for the same purpose.  The overlap in the type of CSM you can create from each of 
their constructors is very tiny to non-existant.

The end result is that it ends up being infeasible to define a PixelInterleaved and a Banded SM that are equals() (not 
impossible, but you'd have to have a very degenerate case like a 1x1 image to make it through the various restrictions 
in the constructors and the offsets and the scanline strides and pixel strides, etc.).  It's really odd in theory, and 
while not a problem in practice it still feels as if it violates an expectation.  The primary restrictions I see getting 
in the way of different classed objects being equals() is that Banded forces a pixel stride of 1 and PixelInterleaved 
enforces that all band offsets are smaller than the scan stride.

So, technically, I don't see any issue with just removing the hash method as the webrev proposes, but I'd like to see 
Phil's reaction to the situation we are in here with respect to intent vs. theory vs. practice vs. developer expectation...

				...jim

On 6/24/16 10:34 AM, Jayathirth D V wrote:
> Hi,
>
> Please find following changes for review in JDK9 :
>
> Bug : https://bugs.openjdk.java.net/browse/JDK-8153943
>
> Webrev : http://cr.openjdk.java.net/~jdv/8153943/webrev.03/
>
> Issue : We have hashCode() method in PixelInterleavedSampleModel and BandedSampleModel, but we don't have corresponding equals() method.
>
> Solution : In PixelInterleavedSampleModel and BandedSampleModel we don't have unique properties that are specific to these subclasses and we have proper equals() & hashCode() method in parent class ComponentSampleModel. So removed hashCode() method present in PixelInterleavedSampleModel and BandedSampleModel.
>
> Thanks,
> Jay
>
> -----Original Message-----
> From: Jim Graham
> Sent: Wednesday, May 04, 2016 2:44 AM
> To: Phil Race
> Cc: 2d-dev at openjdk.java.net
> Subject: Re: [OpenJDK 2D-Dev] Review Request for JDK-8153943 : In java.awt.image package some of the classes are missing hashCode() or equals() method
>
> Yes, the equals/hashcode chapter in Effective Java includes rules about ignoring fields that can be calculated from other fields (which applies to most internal fields).  Basically, only things in the constructors tend to be good candidates for equals/hashcode...
>
> 			...jim
>
> On 5/3/2016 2:00 PM, Phil Race wrote:
>> On 04/26/2016 04:10 PM, Jim Graham wrote:
>>> The ComponentColorModel implementation is a meaningless wrapper
>>> around super.equals/hashCode().  Why does it not test CCM-specific fields?
>>
>> It should although this looks like it is more than just running
>> through all the fields and testing them for equality.
>> eg it looks like "initScale()" should be called if necessary before
>> determining equality and the field "needScaleInit" is not one that has
>> to be directly included in the equality comparison.
>>
>> -phil.
>>
>>
>>
>>>
>>> The ComponentSampleModel.hashCode() method should be upgraded based
>>> on the recommendations in Effective Java like the other methods here.
>>>
>>> PixelInterleavedSampleModel and BandedSampleModel also have a
>>> meaningless override of super.equals/hashCode().
>>>
>>> And all of these classes suffer from casting to the specific type
>>> before verifying its class as I mentioned in the ICM.equals() review...
>>>
>>>             ...jim
>>>
>>> On 4/25/16 2:31 AM, Jayathirth D V wrote:
>>>> Hi Jim,
>>>>
>>>> I have made changes to include check for class equality in base
>>>> class and use super.equals() from subclasses.
>>>> Please find updated webrev for review :
>>>> http://cr.openjdk.java.net/~jdv/8153943/webrev.02/
>>>>
>>>> Change related to ColorModel is present in JDK-7107905 review.
>>>>
>>>> Thanks,
>>>> Jay
>>>>
>>>> -----Original Message-----
>>>> From: Jim Graham
>>>> Sent: Saturday, April 23, 2016 7:30 AM
>>>> To: Phil Race; Jayathirth D V
>>>> Cc: 2d-dev at openjdk.java.net
>>>> Subject: Re: [OpenJDK 2D-Dev] Review Request for JDK-8153943 : In
>>>> java.awt.image package some of the classes are missing hashCode() or
>>>> equals() method
>>>>
>>>> This is actually a pretty nasty issue that Joe Darcy also brought up
>>>> in the CCC review.
>>>>
>>>> In order to have symmetric testing of .equals(), we pretty much have
>>>> to enforce this test at all levels, including in the original
>>>> ColorModels.equals() method.  If we don't enforce this in
>>>> CM.equals(), then we could run ccm.equals(othercm) and it would
>>>> return false because the class is wrong, but turning it around and
>>>> testing
>>>> othercm.equals(ccm) would succeed because it doesn't enforce the
>>>> class equality.
>>>>
>>>> So, I'd recommend that CM.equals() tests getClass() == getClass() at
>>>> the base level and then all others will use super.equals() and get
>>>> the same protection.  It means you can't have a subclass of CCM be
>>>> "equals" to a different subclass of CCM, but that's an unfortunate
>>>> issue with equals needing to honor symmetry...  :(
>>>>
>>>>             ...jim
>>>>
>>>> On 4/20/2016 10:17 AM, Phil Race wrote:
>>>>> Hi, You removed the following test in CCM.java : 2941 if
>>>>> (obj.getClass() != getClass()) {
>>>>> 2942 return false;
>>>>>
>>>>> 2943         }
>>>>>
>>>>> What this means is that before your change an instance of a
>>>>> subclass of CCM would never be equals() to any direct
>>>>> instantiatation of CCM but after your change it can be. I suspect
>>>>> the condition was there on purpose.
>>>>>
>>>>> -phil.
>>>>>
>>>>> On 04/20/2016 05:45 AM, Jayathirth D V wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>>
>>>>>>
>>>>>> _Please review the following fix in JDK9:_
>>>>>>
>>>>>>
>>>>>>
>>>>>> Bug : https://bugs.openjdk.java.net/browse/JDK-8153943
>>>>>>
>>>>>>
>>>>>>
>>>>>> This is subtask of
>>>>>> https://bugs.openjdk.java.net/browse/JDK-6588409
>>>>>>
>>>>>>
>>>>>>
>>>>>> Webrev : http://cr.openjdk.java.net/~jdv/8153943/webrev.00/
>>>>>> <http://cr.openjdk.java.net/%7Ejdv/8153943/webrev.00/>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Issue : Some of the java.awt.image classes are missing either
>>>>>> equals() or hashCode() method.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Solution : Add missing equals() or hashCode() methods.
>>>>>>
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Jay
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>

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