[SPARK-6990][BUILD] Add Java linting script; fix minor warnings

This replaces https://github.com/apache/spark/pull/9696

Invoke Checkstyle and print any errors to the console, failing the step.
Use Google's style rules modified according to
https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide
Some important checks are disabled (see TODOs in `checkstyle.xml`) due to
multiple violations being present in the codebase.

Suggest fixing those TODOs in a separate PR(s).

More on Checkstyle can be found on the [official website](http://checkstyle.sourceforge.net/).

Sample output (from [build 46345](https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/46345/consoleFull)) (duplicated because I run the build twice with different profiles):

> Checkstyle checks failed at following occurrences:
[ERROR] src/main/java/org/apache/spark/sql/execution/datasources/parquet/UnsafeRowParquetRecordReader.java:[217,7] (coding) MissingSwitchDefault: switch without "default" clause.
> [ERROR] src/main/java/org/apache/spark/sql/execution/datasources/parquet/SpecificParquetRecordReaderBase.java:[198,10] (modifier) ModifierOrder: 'protected' modifier out of order with the JLS suggestions.
> [ERROR] src/main/java/org/apache/spark/sql/execution/datasources/parquet/UnsafeRowParquetRecordReader.java:[217,7] (coding) MissingSwitchDefault: switch without "default" clause.
> [ERROR] src/main/java/org/apache/spark/sql/execution/datasources/parquet/SpecificParquetRecordReaderBase.java:[198,10] (modifier) ModifierOrder: 'protected' modifier out of order with the JLS suggestions.
> [error] running /home/jenkins/workspace/SparkPullRequestBuilder2/dev/lint-java ; received return code 1

Also fix some of the minor violations that didn't require sweeping changes.

Apologies for the previous botched PRs - I finally figured out the issue.

cr: JoshRosen, pwendell

> I state that the contribution is my original work, and I license the work to the project under the project's open source license.

Author: Dmitry Erastov <derastov@gmail.com>

Closes #9867 from dskrvk/master.

2 files changed, 2 insertions, 2 deletions

diff --git a/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeExternalRowSorter.java b/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeExternalRowSorter.java
index 3986d6e18f..352002b349 100644
--- a/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeExternalRowSorter.java
+++ b/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeExternalRowSorter.java
@@ -51,7 +51,7 @@ final class UnsafeExternalRowSorter {
   private final PrefixComputer prefixComputer;
   private final UnsafeExternalSorter sorter;
 
-  public static abstract class PrefixComputer {
+  public abstract static class PrefixComputer {
     abstract long computePrefix(InternalRow row);
   }
 
diff --git a/sql/catalyst/src/main/java/org/apache/spark/sql/types/SQLUserDefinedType.java b/sql/catalyst/src/main/java/org/apache/spark/sql/types/SQLUserDefinedType.java
index df64a878b6..1e4e5ede8c 100644
--- a/sql/catalyst/src/main/java/org/apache/spark/sql/types/SQLUserDefinedType.java
+++ b/sql/catalyst/src/main/java/org/apache/spark/sql/types/SQLUserDefinedType.java
@@ -41,5 +41,5 @@ public @interface SQLUserDefinedType {
    * Returns an instance of the UserDefinedType which can serialize and deserialize the user
    * class to and from Catalyst built-in types.
    */
-  Class<? extends UserDefinedType<?> > udt();
+  Class<? extends UserDefinedType<?>> udt();
 }