ARROW-1815: [Java] Rename MapVector to StructVector

[BryanCutler]

This renames the MapVector to StructVector and NullableMapVector to NullableStructVector. Related classes have also been renamed as well as APIs making reference to "Map" meaning a struct. The MinorType.Map is also changed to MinorType.STRUCT.

[BryanCutler]

@siddharthteotia this was pretty much straightforward renaming, so hopefully it is not too disruptive to you downstream. Let me know if I can help run Dremio tests if needed. cc @icexelloss

[icexelloss]

@BryanCutler Are these the final names for the vectors? I think we were thinking about calling it StructVector instead of NullableStructVector but forgot how the discussion ended.

[BryanCutler]

Are these the final names for the vectors? I think we were thinking about calling it StructVector instead of NullableStructVector but forgot how the discussion ended.

I think at one point we agreed that we would eventually merge the 2 and just have StructVector, but our most recent discussion was just about changing the name, so I kept it at that in order not to be too disruptive. If @siddharthteotia is ok with merging the 2 now, I can do that now or make a followup PR.

[BryanCutler]

I believe there was some issues with runtime code optimizations downstream in Dremio that would need to be verified before merging the 2.

[icexelloss]

Aha I see. Sounds good to me. @jacques-n I think the last discussion was left at

https://docs.google.com/document/d/1n4qjO20wZyS7wSpISgYdIVuD22zstLgP-7gXxS5_7_E/edit?disco=AAAABdbHcQQ

Do you think we can proceed with this patch? Or do sth different?

[wesm]

I think @jacques-n is on vacation this week, but we can wait for @siddharthteotia to advise

[siddharthteotia]

I am fine with the name changes of MapVector to StructVector. However, I would request to keep the NullableStructVector as of now since there are potential impacts on reader/writer I believe.

I would like to patiently rebase Dremio on Arrow master -- we are yet to the changes w.r.t removal of non-nullable vectors and renaming of other vectors. Once that is done, we can hopefully remove NullableStructVector too.

[icexelloss]

Hey folks, where do we stand on this? @siddharthteotia are we ok with the changes to MINOR type and reader/writer interface?

[jacques-n]

It seems like we should be changing from NullableMapVector => StructVector and MapVector => NonNullableStructVector to be consistent with other names, no? Or do we discuss that as a follow along change?

[BryanCutler]

I made the above name changes to the vector classes, so we now have StructVector and NonNullableStructVector. I looked into changing the reader/writer apis to match (currently we have SingleStructWriter and NullableStructWriter) and it gets a little messy because there is already an interface named StructWriter. So I left those as is and maybe they could be looked at whenever we end up merging the 2 vector classes. What do you think about this @jacques-n @siddharthteotia @icexelloss ?

[jacques-n]

LGTM +1

[wesm]

Cool, will merge. Thanks all!

[BryanCutler]

Thanks @jacques-n and @wesm!