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

Jayathirth D V jayathirth.d.v at oracle.com
Thu Jul 28 11:42:15 UTC 2016


We can follow the same approach of what we did in ColorModel.equals() for JDK-7107905.

We can use getClass() check in ComponentSampleModel.equals() to differentiate between BandedSampleModel and PixelInterleavedSampleModel in rare cases where they can have same values and remove the need for equals() & hashCode() methods in these subclasses.
We can update the specification of ComponentSampleModel.equals() to mention what properties we are checking and what its subclasses to follow as we did in ColorModel.

Please find webrev for the above changes:


-----Original Message-----
From: Jim Graham 
Sent: Thursday, June 30, 2016 4:22 AM
To: Phil Race
Cc: Jayathirth D V; 2d-dev at openjdk.java.net
Subject: Re: [OpenJDK 2D-Dev] Review Request for JDK-8153943 : In PixelInterLeavedSampleModel and BandedSampleModel we dont need hashCode() method

Hi Phil,

The following 2 statements are factually correct ignoring what we plan to do about these classes:

- If they don't override equals() then they shouldn't override hash

- Their raw casts were wrong (and the fact that nobody ever noticed that is a good indication that these methods have probably not been used in the wild).

What I'm waffling on is that while the equals() methods appeared functionally useless, in practice they can either block or allow disparate class objects from being equal to each other and I'm thinking that one could argue that it makes strategic sense to enforce the correct class type (using instanceof) to match the behavioral expectations of a developer.

Since I sent that out I also checked a similar case in the Number subclasses and an Integer and a Long will not evaluate to equals even if they hold the same cardinal value.  Arguably that's even a stronger case of when 2 disparate objects might want to be seen as equals(), but we don't allow it there.

Arguably, the override of hash is still unnecessary since it would be fine to allow disparate classes to have the same hash value from the super class - that doesn't violate any contracts.  You can't allow it to inherit from Object.hashCode() since that hash value is very strict, but it would be OK to override the somewhat "property oriented" hash code from their immediate superclass.  I would lean towards leaving it in if we decide to override equals() just to avoid the argument.

Given the fact that these equals() methods have obviously not been used much, I don't have a strong opinion between "saving code by just not overriding them" and "making the different subclasses have unique identities by overriding and preventing them from being equal to each other".

In the end, if we do fix the equals() for these classes, we should document why they are overriding equals() even if it appears to do not really value added testing compared to the super class...


On 06/29/2016 02:29 PM, Phil Race wrote:
> To remind myself and others .. this review started out with changes to 
> the ColorModel classes mixed in.
> Those were separated out making it easier to review just the 
> SampleModel hierachy here.
> Jim observed (see some way below for the context) :-
>  > PixelInterleavedSampleModel and BandedSampleModel also have a  > 
> meaningless override of super.equals/hashCode().
> I think that comment is from when the webrev for those files looked 
> like this :-
> http://cr.openjdk.java.net/~jdv/8153943/webrev.02/src/java.desktop/sha
> re/classes/java/awt/image/BandedSampleModel.java.sdiff.html
> http://cr.openjdk.java.net/~jdv/8153943/webrev.02/src/java.desktop/sha
> re/classes/java/awt/image/PixelInterleavedSampleModel.java.sdiff.html
> So Jim, are you suggesting a preferred option is to go back to that 
> proposal - at least for the equals methods ?
> [Caveat : the proposed equals method there should test instanceof 
> before doing a cast but I actually don't know why it needs the cast 
> anyway]
> The hashCode() could be left alone in that case.
> But if the equals methods are not added as there, then I think we do 
> need to remove the
> hashCode() since it could be different even if (albeit only in a 
> degenerate case),  whilst instances (of the different classes) could 
> compare as equal.
> But in either case we need to look to the super-class which is lacking 
> any documentation of its own describing what makes two instances 
> equal.
> We could try to explain there what might otherwise be surprising.
> -phil.
> On 06/28/2016 01:40 PM, Jim Graham wrote:
>> Still hoping to hear an opinion from Phil on this...
>> The alternative is to add equals() overrides in the subclasses that 
>> just do "obj instanceof <myclass> && super.equals()" which would only 
>> matter in some specific somewhat degenerate cases.
>> The intent would be that even though the layout and pixel fetching 
>> behavior for a specific configuration of PISM and BSM are identical, 
>> they are philosophically not the same and so should not evaluate as 
>> equals()...
>> ...Or, should they?
>>             ...jim
>> On 6/27/16 4:05 PM, Jim Graham wrote:
>>> 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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