Code review: 7012540 (java.util.Objects.nonNull() incorrectly named)

[Brian Goetz]

The only reason we're even having this discussion now -- as we're well 
past freeze for 7 -- is to prevent the current situation from getting 
carved into stone, where we have a nonNull() precondition-enforcing 
method in Objects.  While the correct name for the 
postcondition-producing version is tangentially relevant to that, the 
short-term goal -- which we keep drifting from -- is renaming the 
precondition-enforcing version so as to *also allow room for* a 
postcondition-producing version later.

Anything more than this is going to get rejected on a "sorry, it's too 
late" basis.

(Its amusing that the goal here was to eliminate a name that was 
confusing because it could apply equally well to two equally valid use 
cases, and this is in fact so confusing that even we cannot be 
consistent about which version we're discussing....)

On 1/26/2011 11:01 AM, David Schlosnagle wrote:
> On Tue, Jan 25, 2011 at 6:33 PM, Brian Goetz<brian.goetz at oracle.com>  wrote:
>>>> Additional notes: After much discussion on core-libs-dev, the name
>>>> requireNonNull() seemed the least objectionable.
>>>
>>> I think requireNonNull(x) is confusing.
>>
>> Remember there's two versions of someModifierNonNull being discussed; the
>> one currently in Objects is the precondition-enforcing variety, not the
>> postcondition-ensuring variety.  Are we talking about the same thing?
>>
>>> For those familiar with the "requires/ensures/modifies" triad of verbs as
>>> a compact way of identifying the preconditions, postconditions, and side
>>> effects of a method or procedure in a comment, a specification, or a more
>>> formal design-by-contract framework, "requires" is just wrong.
>>>
>>> When analyzing the invocation of foo in your example, the non-nullity of
>>> s and t are preconditions of foo and therefore postconditions of the
>>> check method.  Naming the check method "requireNonNull" suggests that
>>> the check method itself has a precondition that its argument be non-null,
>>> when in fact it's the check method's postcondition which ensures that
>>> property.
>>>
>>> Since postconditions are labeled "ensures" in the "r/e/m" triad, this
>>> method should be named "ensureNonNull".
>>
>> Right, there's precedent for "ensureXxx" methods to actually change the
>> state of things to ensure the postcondition, such as ensureCapacity()
>> methods in the collection implementations.  Given that a part of the
>> motivation for this change was to leave room in the namespace for both the
>> precondition-enforcing variety (barf on null) and the postcondition-ensuring
>> variety, aka the carpet-sweeping variety ("if it is null, make it non-null")
>> ensureNonNull sounds a lot more like the the carpet-sweeping version than
>> the version being discussed (barf on null).
>>
>> The r/e/m framework seems to support require for the throwing version (as
>> implemented by this patch): for the throwing version, non-nullity is a
>> precondition of the check method (if the condition is not met, error),
>> whereas for the carpet-sweeping version, it is a postcondition of the check
>> method (if the check method can come up with a reasonable default value).
>>   (It happens to be a postcondition of both, but the significant behavior and
>> use of the throwing version currently in Objects is to enforce an error when
>> the precondition is not met.)  Therefore:
>>
>>   requireNonNull(x) ->  throw if x == null
>>   ensureNonNull(x)  ->  convert x to a non-null value if null
>>
>> seems like the right taxonomy.
>>
>
> If you're still open to other possible names for the requireNonNull
> method, based on some of the evaluation comments on the highest rated
> RFE [1] would you prefer assumeNonNull that throws
> NullPointerException when the assumption is violated, otherwise
> returns the specified object reference? I honestly don't have a strong
> opinion between either requireNonNull or assumeNonNull, but I think it
> is at least a small step toward a more comprehensive preconditions
> API. As I mentioned before, I'd love to see something along the lines
> of Guava's Preconditions or Apache commons-lang's Validate APIs as
> part of the JDK, but that is probably best left to JDK 8 to better
> flesh out.
>
> [1]: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4449383
>
> - Dave