PROPOSAL : Specify nullable argument (original) (raw)

Reinier Zwitserloot reinier at zwitserloot.com
Tue Mar 17 17:27:33 PDT 2009

Adding non-nullity to the type system would be a great boon, IMO, but
its far more complicated than just having a @NonNull annotation.

I've written up a proposal a while ago here: http://www.zwitserloot.com/2008/10/23/non-null-in-static-languages/

Suffice to say it gets very complicated because you need 4 nullity
states (Never-Allows-Null, Definitely-Allows-Null, Might-Allow-Null,
Legacy (like raw generics, for interop with old code), AND you need
a separate process to promote or demote generics (So that you can say:
Eventhough this is a Map that maps NonNull String to NonNull String,
if you call get() on this, you get a @Nullable String back, because
null will flow out if I can't find the key. In other words, the 'V'
generics parameter needs to be demoted to @Nullable V somehow).

Read the blog entry for a more elaborate explanation with examples.

I'd love something like that in java, but the proposal as it stands
needs a source 1.7; keyword (because having no modifier is taken as
'never null', which is obviously not backwards compatible), and is
very complicated, so: Definitely not project coin material.

Incidentally, I think jsr305 is fundamentally broken. It doesn't have
the states needed to convey nullity info; it needs at least 2
annotations, probably 3 to be able to differentiate between code that
simply didn't specify anything, and code that did, instead of the one
annotation that jsr305 gives.

--Reinier Zwitserloot

On Mar 17, 2009, at 21:41, Neal Gafter wrote:

Did you intend this to be a complete proposal?

Do you like this better than @NonNull, which will probably be included in Java 7 under jsr 305? On Tue, Mar 17, 2009 at 1:25 PM, Olivier Chorier <lapsus63 at gmail.com> wrote: Example :

public void getListOfProducts(Company owner, !String facultativeName) { .... } The '!' should prevent the developper that the argument is able to be null. Invoking getListOfProducts(null, "name") could throw a compilation error/warning. Or maybe better : public void getListOfProducts(!Company owner, String facultativeName) { .... } Here, the '!' indicates a mandatory argument (not null).

However, I don't know if the '!' marker could be the most appropriate.

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