Add Predicate.of(), Consumer.of(), etc.

Attila Szegedi attila.szegedi at
Wed May 6 11:58:58 UTC 2015

On May 6, 2015, at 12:37 PM, Paul Sandoz <paul.sandoz at> wrote:
> On May 2, 2015, at 11:31 PM, Remi Forax <forax at> wrote:
>> Hi all,
>> today, I stubble on a variant of JDK-8050818 [1],
>> trying to call negate() on a lambda which is not yet a Predicate (due to target typing) which requires either to cast the lambda to a Predicate and everybody knows that cast are evil or to add a local variable.
>> I think there is a way to fix that, it's not very orthodox so I let you judge.
>> The idea is to introduce a static method 'of' on Predicate,
>> class Predicate<T> {
>>   ...
>>   public static <T> Predicate<T> of(Predicate<T> predicate) {
>>     return predicate;
>>   }
>> }
>> so one can write:
>> stream.filter(Predicate.of(String::isEmpty).negate())
>> compared to
>> stream.filter(((Predicate<String>)String::isEmpty).negate())
>> so the question is, does it it worth the cost to introduce a static method that basically do nothing on all functional interfaces of java.util.function.
> That does have the virtue of not adding a static method per operation but i still cannot bring myself to add such methods as a work around for seems like a compiler inference problem (albeit one in which might be very tricky or not possible to solve).

I think it is firmly in the category of “very tricky”, but probably still possible, albeit (from my point of view) undesirable.

String::isEmpty will not, on its own, have a method named .negate(); the compiler can’t really infer the programmer’s intent to make it into a Predicate, not without a fairly high level reasoning (filter needs a Predicate; should the compiler perform an exhaustive search of the visible functional interfaces to see which one has a method with signature “Predicate negate()”?). So, yeah, it seems like in this case the programmer needs to communicate the intent. 

I think inference would be *possible*, but it’d be expensive, and would still allow for either ambiguities or accidentally matching something unexpected, so I think it would also be undesirable.

I don’t have an opinion of whether we want an “of” static method on Predicate, as then it would have to indeed be introduced on many other interfaces. It’s more appealing than a cast, certainly; especially since the cast apparently needs to specify the explicit type parameter too.

> In some respects i wonder if the default methods on the functional interfaces are an attractive nuisance.

Meaning, if .negate(Predicate) were a static method on the Predicate class instead of a default method, then stream.filter(Predicate.negate(String::isEmpty)) would be possible? Yeah…


> Paul.

More information about the core-libs-dev mailing list