ARROW-6021: [Java] Extract copyFrom and copyFromSafe methods to ValueVector interface

[liyafan82]

Currently we have copyFrom and copyFromSafe methods in fixed-width and variable-width vectors. Extracting them to the common super interface will make it much more convenient to use them, and avoid unnecessary if-else statements.

[codecov-io]

Codecov Report

Merging #4931 into master will increase coverage by 2.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4931      +/-   ##
==========================================
+ Coverage    87.5%   89.64%   +2.13%     
==========================================
  Files         998      664     -334     
  Lines      142085    98021   -44064     
  Branches     1418        0    -1418     
==========================================
- Hits       124336    87872   -36464     
+ Misses      17387    10149    -7238     
+ Partials      362        0     -362

Impacted Files	Coverage Δ
r/src/recordbatch.cpp
r/R/Table.R
js/src/util/fn.ts
go/arrow/array/bufferbuilder.go
r/src/symbols.cpp
rust/datafusion/src/execution/projection.rs
rust/datafusion/src/execution/filter.rs
rust/arrow/src/csv/writer.rs
rust/datafusion/src/bin/main.rs
go/arrow/ipc/file_reader.go
... and 327 more

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[pravindra]

there's already an impl of copyFrom in UnionVector, right ?

https://github.com/apache/arrow/blob/master/java/vector/src/main/codegen/templates/UnionVector.java#L367

[liyafan82]

Nice catch. Thank you @pravindra

[pravindra]

things can go bad if the caller passes a fixed-width vector in the 'from'.

[liyafan82]

Good question.
Things will go wrong is the source and target are of different vector types. Even if both are fixed-width vectors, things can go wrong (e.g. from a IntVector to a Float4Vector).

One solution is to check types each time, however, this may have performance penalty, as copyFrom is a frequently called operation. So current assumption is that the caller should make sure the source and target are of the same type.

[pravindra]

can we add something cheap, like checking the minorType ?

[liyafan82]

Good suggestion. Thank you @pravindra

[pravindra]

thanks for the change, @liyafan82