Return unmodifiable string set from preferences

[NightlyNexus]

https://developer.android.com/reference/android/content/SharedPreferences.html#getStringSet(java.lang.String,%20java.util.Set%3Cjava.lang.String%3E)

This can cause subtle errors and ConcurrentModificationExceptions if the set is mutated by the caller.
This is a breaking change, but I thought I'd propose it, anyway. :)
Also, it could just return a mutable copy (new LinkedHashSet<>() instead of unmodifiableSet()) to be a compatible change.

[NightlyNexus]

oh, it's going to throw an npe. will fix.

[f2prateek]

Thanks! Changes LGTM.

Behaviour wise, not 100% sure if it's the responsibility of the library to do this. Might be equally surprising for a user to see we're wrapping the set returned by the preferences.

[f2prateek]

(I'm not too worries about the breaking changes part since v2 is relatively new, and might need some breaking changes with Flowables and nulls anyway)

[NightlyNexus]

I think API calls for collections should normally return unmodifiable collections. Returning mutable collections is a surprising way to share state (and it's just an oversight/error for the Android sdk to do so).
That's my feeling haha.

[NightlyNexus]

(I realized it's actually not a breaking change, since the default value is Collections.emptySet(), currently.)

[f2prateek]

@NightlyNexus sorry forgot to follow up here. What's behaviour if you call SharedPreferences#getStringSet(). It returns a mutable value, correct?

[NightlyNexus]

That's actually the whole of the problem.
It's "mutable," but it's not supposed to be mutable, leading to subtle bugs. (I eliminated ConcurrentModificationExceptions from the apps I worked on by fixing this in my fork.)

From the doc:

Note that you must not modify the set instance returned by this call.

Just sad legacy behavior in AOSP that it doesn't return an unmodifiable set, I believe.

[f2prateek]

Got it. Ok let's add a small note about this in javadoc and

[NightlyNexus]

Done. I just made a quick edit to the javadoc and am never offended if you want to tweak any wording, etc. haha.

[f2prateek]

LGTM!