On Thu, Sep 13, 2012 at 01:10:41AM +0200, Gilles Sadowski wrote:
> Hello.
>
> [For those who don't wish to read the whole post, please at least go towards
> the end and indicate your preferred option. Thanks.]
>
> [... long post skipped ...]
>
> Back to square one, with 3 fully consistent alternatives:
> 1. CM to always check for null? Then "NullArgumentException" inheriting from
> "MathIllegalArgumentException" is fine because we promise to the user that
> no NPE will ever propagate through the CM layer. [Breaking that promise
> is a CM bug.]
> 2. CM to never check for null? Then we delete class "NullArgumentException".
> Users are warned by the general policy: "Do not pass null unless it is
> explicitly documented to be allowed." A bug will lead to the JVM raising
> a NPE.
> 3. CM to sometimes check for null? Then "NullArgumentException" should
> inherit from "NullPointerException" because the user will sometimes see
> "NullArgumentException" (when CM checks) and sometimes NPE (when CM does
> not check) and both should thus belong to the same hierarchy (from the
> user's point-of-view, there is no reason to handle them in different
> ways since it's the exact same bug).
> For the user, the consequence will be similar to alternative 2, with
> sometimes more information about the failure and sometimes (marginally)
> earlier detection.
My preference would be alternative 2 but I'd be happy with alternative 3 too
(as a compromise to allow checks for people who need more time to get rid
of the habit :-).
With those additional arguments, for good measure:
(a)
---
Thrown when an application attempts to use null in a case where an object is
required. These include:
Calling the instance method of a null object.
Accessing or modifying the field of a null object.
Taking the length of null as if it were an array.
Accessing or modifying the slots of null as if it were an array.
Throwing null as if it were a Throwable value.
Applications should throw instances of this class to indicate other illegal
uses of the null object.
---
[This is the Javadoc for NPE. Please note the last sentence.]
(b)
"null" is not a representation of an application-domain concept, it is a low
low low level literal (the unique element of the null-type) and exists solely
to be the value of unitialized pointers (not even "reference"). It is only
perfectly right that attempts to use "null" are set apart from other bugs
(that acquire meaning only wrt to higher level preconditions).
(c)
http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Preconditions.html#checkNotNull(T)
Gilles
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