add Nonnull and Nullable annotations to ContextRun function

[hiteshsharma]

This change adds support for javax.annotation.Nonnull and javax.annotation.Nullable annotations for Parseq API.
This change is a follow up on discussion #205
This change only adds annotations for Task.contextRun method. Once this is accepted, I'll create more changes for adding annotations to other functions.
Annotations are decided based on current code usage.
@mchen07

[mchen07]

I have a little reservation that you picked this ContextRun function, as typical ParSeq users, these functions are too internal to them. I would encourage them to always try to use methods provided in Task class. So if we would like to add Nonnull or Nullable, I may prefer to start from there.

[hiteshsharma]

@mchen07 agree. I just picked the first public function in Task.java to demonstrate the change. I plan to enable these for entire public API one function at a time. If you think beginning with some other function would be a good idea, I can send an update.

[mchen07]

I may prefer that we start with adding annotation to Task interface.

[hiteshsharma]

@mchen07 reverted original commit and added @Nonnull annotations to map functions in Task interface. If this looks good, will do this for all default functions.
let me know what you think.

[mchen07]

why do we need to add this findbugs dependency? As we discussed earlier, I hope that this annotation should not depend on one particular static analysis tools.

[hiteshsharma]

javax.annotation package is part of findbugs. This is just lib import, we don't need to run findbugs to use these annotations but if you're not willing to import lib as well then this change can't be made.
In my opinion importing this lib should not have any impact and it's a very standard practice.
let me know what you think.

[hiteshsharma]

@mchen07 bump

[mchen07]

Sorry for the delay. I am still struggling whether we should introduce such @nullable annotation where different IDE may have different support (for example, IntelliJ has its own @nullable support, which is different from javax.annotation, see https://www.jetbrains.com/help/idea/nullable-and-notnull-annotations.html), or instead we should use java Optional to indicate this, where we don't depend on the implementation of static code analysis tool.

[hiteshsharma]

Hey @mchen07 documentation doesn't mentions it (probably old) but IntelliJ supports it. You can go to Intellij Preferences -> Editor -> Inspections -> Java -> Probable bugs -> @NotNull/@Nullable problems -> Configure annotations
There you can see javax.annotation.Nullable and Notnull too

[mchen07]

Thanks for the tip @hiteshsharma. Have you seen this: spotbugs/spotbugs#180? Recently spotbugs is preferred over findBugs, will this have side effect on that trend?