Varargs cause compiler warnings

[samuelgruetter]

I'm trying to use the concat operator, but I can't use it without getting a compiler warning. Here's my code:

Observable<HorrorMovie> horrorMoviesWithException() {
    Observable<HorrorMovie> movies = Observable.from(new HorrorMovie());
    Observable<HorrorMovie> ex = Observable.error(new Exception("test")); 
    return Observable.concat(movies, ex); // (WARNING)
}

I get the warning "Type safety: A generic array of Observable<CovarianceTest.HorrorMovie> is created for a varargs parameter".

This problem was discussed on stackoverflow and this bug report explains why it's not going to be fixed.

[samuelgruetter]

The same problem also occurs with merge, mergeDelayError, and if arguments with generic type are passed to from and startWith, they are also affected.

[samuelgruetter]

Unless someone shows me how to get rid of this warning without using @SuppressWarnings("unchecked"), I would suggest that we remove the varargs methods and add instead

• an overloaded method for each arity
• a method taking an Iterable (will be used by the Scala adaptor)

[abersnaze]

Like the zip operator?

Sent from my iPhone

On Sep 9, 2013, at 1:50 AM, samuelgruetter [email protected] wrote:

Unless someone shows me how to get rid of this warning without using @SuppressWarnings("unchecked"), I would suggest that we remove the varargs methods and add instead

an overloaded method for each arity
a method taking an Iterable (will be used by the Scala adaptor)
—
Reply to this email directly or view it on GitHub.

[samuelgruetter]

yes

[benjchristensen]

That issue with varargs is definitely one of the weaknesses of generics.

How about if we had overloads for up to 9 arities, and then on that one have a 10th vararg for scenarios were people truly do want/need more? That way it would only affect someone using beyond 9 args?

[samuelgruetter]

I don't quite get what you mean by "a 10th vararg for scenarios..." but I'd also be fine with overloads for up to 9 arguments, plus another overload which takes only 1 argument of type Collection<Observable<T>> or Iterable<Observable<T>>, so if people want more than 9 arguments, they have to construct a list containing the arguments and pass this list to concat.

[benjchristensen]

I meant something like this ...

concat(T t1, T t2)
... etc ...
concat(T t1, T t2, T t3, T t4, T t5, T t6, T t7, T t8)
concat(T t1, T t2, T t3, T t4, T t5, T t6, T t7, T t8, T t9)
concat(T t1, T t2, T t3, T t4, T t5, T t6, T t7, T t8, T t9, T... rest)

I will start refactoring vararg method signature later today or tonight (US Pacific time) unless someone else is already doing it. If one of you already are tackling this please let me know here so we don't replicate effort.

[benjchristensen]

Closing as I have removed all varargs from Observable.

[benjchristensen]

The @SafeVarargs annotation allows us to solve this. Trying to figure out if we can leverage this Java 7 feature without breaking Android support (which is obviously more important).