2.x: Add @NonNull annotation for Function/Predicate/etc.

One of the larger tasks when transitioning from 1 to 2 is removing the use of null. Right now there’s no automated way to do it, but if we had @NonNull then we’d be able to use static analysis to find where we’re returning null when we shouldn’t be.

I’m not sure the best way to go about it - if we should include jsr305, use Java 8’s NonNull, create our own, or something else.

[dlew]

Hmm, I’m most concerned about the functional interfaces, actually. Those are the ones where the app may accidentally crash during execution unexpectedly.

Here’s a contrived example:

void main() {
    Observable.just("Dan")
        .map(new Function<String, String>() {
            @Override
            public String apply(String s) throws Exception {
                return usuallyDependableMethod(s);
            }
        })
        .subscribe();
}

String usuallyDependableMethod(String name) {
    return (Math.random() < .95) ? "Hello " + name : null;
}

I can imagine a lot of circumstances where coders don’t even realize that usuallyDependableMethod() could sometimes return null (when it’s not such an obvious problem). That’s where static analysis would really help out.

[dlew]

It looks like Java 8 doesn’t actually include it - it’s part of a checker framework. More details here.

Regardless, I’m specifically targeting IntelliJ’s “Constant Conditions & Expressions” inspection and it looks like you can tell IntelliJ about which annotations should be considered non-null / nullable. So it would work if RxJava had its own @NonNull annotation included with the library (as long as developers are willing to do a little configuration on their end to accept it).

The only downside to not using jsr305 is that we can’t use something like @ParametersAreNonnullByDefault which would be less verbose.

If people are on board with this, I could start implementing this (but it’s a lot of busywork so I don’t want to bother unless people want it).