Implementation of == and Object::equals

[Roger Riggs]

Hi John,

This is covered under discussions of ==, specified in the VM spec as 
implemented by the "acmp" bytecode
and described as "same"ness or "substitutability" on the 
valhalla-spec-experts archive.

The discussion of substitutability for float and double along with more 
general principles were discussed in February 2024.
See the archives for details: 
https://mail.openjdk.org/pipermail/valhalla-spec-experts/2024-February/subject.html

Regards, Roger

On 4/7/24 10:30 PM, John Bossons wrote:
> I'm not sure that this is the right place to raise this, but don't 
> know where else to do so.
>
> For identity types, == is simply a test of reference equality and so 
> is a useful first test in an overriding type-specific equals method.
>
> For value types, the relationship is more subtle, since == is a test 
> of value equality, not reference equality. I suggest it be implemented 
> as an operator causing Object::equals to be invoked, so that if 
> Object::equals is overloaded (e.g. to first test equality of hash 
> codes or otherwise modify the default equals test), the overloaded 
> method is automatically invoked when == is used.
>
> This would mean that Object::equals implements the == operator, rather 
> than the reverse, so that a specification that a == b always means 
> a.equals(b), whether implemented by the default test or by an 
> overriding type-specific method (that e.g. tests for equality of 
> identity object field values). I believe this would make value types 
> (especially value trees) easier to use.
>
> John
>
> -- 
> Phone:  (416) 450-3584 (cell)
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