PROPOSAL : Specify nullable argument

Reinier Zwitserloot reinier at
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:

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> 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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