[Java] Add get to BaseIntVector interface

[asfimport]

1. In the set method should not use long as parameter. It is hardly the case that there are more than 2^32 distinct values in a dictionary. If it really happens, maybe it means we should not have used dictionary in the first place. 

2. In addition to the get method, there should also be a set method. 

Reporter: Liya Fan / @liyafan82
Assignee: Ji Liu / @tianchen92

PRs and other links:

• GitHub Pull Request #4859

_{Note: This issue was originally created as ARROW-5918. Please see the migration documentation for further details.}

[asfimport]

Micah Kornfield / @emkornfield:
For #1, the baseIntVector isn't just for dictionaries, its a general interface which is why it was changed to long.

[asfimport]

Ji Liu / @tianchen92:
[~emkornfield@gmail.com]

Sure, For #1 you are right. 

For #2 I think add a get method is needed which could be used in some places like DictionaryEncoder#decode:

for (int i = 0; i < count; i++) {
Object index = indices.getObject(i);
if (index != null) {
int indexAsInt = ((Number) index).intValue();
if (indexAsInt > dictionaryCount) {
throw new IllegalArgumentException("Provided dictionary does not contain value for index " + indexAsInt);
}
transfer.copyValueSafe(indexAsInt, i);
}
}

[asfimport]

Liya Fan / @liyafan82:
[~emkornfield@gmail.com]. I see. That makes sense. Thanks for the clarification.

[asfimport]

Micah Kornfield / @emkornfield:
@tianchen92 it seems reasonable to add a getLong to me, rephrased the JIRA title

[asfimport]

Micah Kornfield / @emkornfield:
or getValueAsLong might be clearer.

[asfimport]

Ji Liu / @tianchen92:
[~emkornfield@gmail.com] Sure, sounds reasonable.

[asfimport]

Micah Kornfield / @emkornfield:
Issue resolved by pull request 4859
#4859