[MINOR][BUILD] Fix Java linter errors

## What changes were proposed in this pull request?

This PR cleans up a few Java linter errors for Apache Spark 2.2 release.

## How was this patch tested?

```bash
$ dev/lint-java
Using `mvn` from path: /usr/local/bin/mvn
Checkstyle checks passed.
```

We can check the result at Travis CI, [here](https://travis-ci.org/dongjoon-hyun/spark/builds/244297894).

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #18345 from dongjoon-hyun/fix_lint_java_2.