[OpenJDK 2D-Dev] Review Request for JDK-7107905: equals() method in IndexColorModel doesnt exist and it relies on ColorModel.equals() which is not strict

[Jayathirth D V]

Hi Jim,

Thanks for the review.
I have made all recommended changes and created new subtask for JDK-6588409(https://bugs.openjdk.java.net/browse/JDK-8153943 ).

Please review updated webrev:
http://cr.openjdk.java.net/~jdv/7107905/webrev.02/ 

Thanks,
Jay

-----Original Message-----
From: Jim Graham 
Sent: Friday, April 08, 2016 12:28 AM
To: Jayathirth D V; Philip Race; Prasanta Sadhukhan
Cc: 2d-dev at openjdk.java.net
Subject: Re: Review Request for JDK-7107905: equals() method in IndexColorModel doesnt exist and it relies on ColorModel.equals() which is not strict

Hi Jayathirth,

This looks good.

One thing to note for efficiency, "instanceof" also checks for null.  It only returns true for non-null objects, so you don't need to explicitly test for null at the top of ICM.equals().  (Though, you should include a
test(s) in the unit test that verifies that no ICM returns true for
equals(null) to be sure.)  You can see that CM.equals() is implemented this way.

Also, for performance, ICM should include a quick "if (this == cm) return true;" check, like CM.equals().  I'd recommend:

- first instanceof
- then == test
- then super.equals()
- finally, test equality of data fields

More comments inline:

On 4/7/16 6:46 AM, Jayathirth D V wrote:
> 	- Yes https://bugs.openjdk.java.net/browse/JDK-6588409 has mentioned the same thing. Can I create a subtask to address java.awt.image changes?

That would be good.

> For now, it would be good to implement hashCode() on ICM now that you are creating an equals() method for it.
>
> 	- I am not completely sure of what is the optimal way of adding hashCode() API so I took help from internet and IDE to make the changes. I am including super.hashCode() as it adds uniqueness to ICM.

You did a great job.  To save time in the future, you should all have copies of the latest version of "Effective Java" by Josh Bloch.  It has a whole chapter on equals/hashCode.  It's a very handy reference for all sorts of good programming practice for the Java language.

> Also, ColorModel.hashCode() is a poor implementation.  It doesn't use the paradigms recommended by Effective Java and looks like it produces poor hashes as a result (just in the first two elements of the hashCode I see a collision due to poor choice of numbers).
> 	- I think since we are not using Prime numbers and it might result in improper hash code. I have made similar changes to hashCode() of ColorModel.

Looks great.

> 	- In the same file itself we are following Java coding guidelines of having braces after if like "if () {". I am not completely sure of including "{" in new line.

You are correct, that matching local code is a good consideration when considering code style.  In this case, though, the indentation on these if statements is an example of what we're trying to avoid so it would be better to fix these particular statements (don't bother fixing the other lines in the file at this point, that can wait until we have to update other parts of the file, but don't propagate a bad style in new code). 
In particular:

Never do this:

     if (some complex test ||
         some additional tests ||
         some additional tests ||
         some additional tests ||
         some continuation of the test) {
         the body of the code;
         more body of the code;
     }
Quick question - where does the body of the if statement start?  Does your eye see it in a fraction of a second or do you have to search for it?

That is the worst option for indenting an if statement with continuation lines.  Never do that in new code.  Do either of the following two versions:

Java Code Style guidelines recommends indenting 8 spaces for conditional
continuations:
     if (some complex test ||
             some additional tests ||
             some additional tests ||
             some additional tests ||
             some continuation of the test) {
         the body of the code;
         more body of the code;
     }

Jim's personal extension to the JCS recommends (and the 2D team historically tended to agree):
     if (some complex test ||
         some additional tests ||
         some additional tests ||
         some additional tests ||
         some continuation of the test)
     {
         the body of the code;
         more body of the code;
     }

Both of those immediately draw the eye to the separating point between the conditional and the body of the code.

> I'd also add a few test cases that test that 2 separately and identically constructed ICM instances are equals() == true, tested with one of each of the constructors...
>
> 	- I have made changes to test case for verifying all constructors with same ICM. Also added verification for hashCode value.

Unfortunately, some of your tests for hashCode make an invalid assumption.  It is technically correct for the hash codes of 2 different objects to be the same regardless of whether they are equals() or not. 
In fact, a perfectly valid implementation of hashCode() could return a constant number and it would be correct from the perspective of the equals/hashCode contract.  (Such code, however, would not be optimal, but it would be valid/correct-to-spec.)  The only condition that is required is that the hash codes match if the objects are equals(), but they are allowed to match if the objects are !equals().  In other words:

	boolean equals1 = (o1.equals(o2));
	boolean equals2 = (o2.equals(o1));
	boolean equalsH = (o1.hashCode() == o2.hashCode());

if (equals1 != equals2) that is an error - we should test this if (equals1 && !equalsH) that is an error - we should test this // No other conditions are an error, no other testing should be done

In particular, the cases where you test the hash codes for being the same on objects that are not intended to be equals() should be deleted. 
  It would be good to also add tests to make sure that they are symmetrically equals (or symmetrically not equals) as well (i.e. 
o1.equals(o2) matches o2.equals(o1) in all cases whether they match or not).

Since it is less than optimal for hash codes to match if the objects are not equal, it might potentially be something to check on, but it should not generate a unit test failure and so should not appear in the unit test for this bug.  Such a "code collision test" would be part of a performance test run periodically for QA, but we have never bothered with hashCode optimization unless a customer submits a complaint about the performance of some object in a hash map due to hash collisions (and I doubt that ColorModel objects are being used in such a manner), so I wouldn't recommend it here.

				...jim