[SPARK-13583] Remove unused imports and add checkstyle rule

• Type: Task
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 2.0.0
• Component/s: DStreams, Spark Core
• Assignee: Dongjoon Hyun
• Reporter: Dongjoon Hyun
• Created: 29/Feb/16 23:33
• Updated: 03/Mar/16 10:13
• Resolved: 03/Mar/16 10:12

Description

After SPARK-6990, `dev/lint-java` keeps Java code healthy and helps PR review by saving much time.

This issue aims to enforce `UnusedImports` rule by adding a `UnusedImports` rule to `checkstyle.xml` and fixing all existing unused imports.

checkstyle.xml

+        <module name="UnusedImports"/>

Unfortunately, `dev/lint-java` is not tested by Jenkins. ( https://github.com/apache/spark/blob/master/dev/run-tests.py#L546 )

This will also help Spark contributors to check by themselves before submitting their PRs.

According to the srowen's comments, this PR also includes the removal of unused imports in Scala code. It will be done by manually because of the following two reasons.

• Scalastyle does not have `UnusedImport` rule yet.
• Scala 2.11.7 has a bug with `-Ywarn-unused-import` option.
  (https://issues.scala-lang.org/browse/SI-9616)

Issue Links

• links to: [Github] Pull Request #11438 (dongjoon-hyun)