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

## What changes were proposed in this pull request?

After SPARK-6990, `dev/lint-java` keeps Java code healthy and helps PR review by saving much time.
This issue aims remove unused imports from Java/Scala code and add `UnusedImports` checkstyle rule to help developers.

## How was this patch tested?
```
./dev/lint-java
./build/sbt compile
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11438 from dongjoon-hyun/SPARK-13583.

15 files changed, 6 insertions, 38 deletions

diff --git a/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala b/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
index 87f3bc31e6..d537b6141c 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/GraphOps.scala
@@ -17,15 +17,10 @@
 
 package org.apache.spark.graphx
 
-import scala.reflect.{classTag, ClassTag}
 import scala.reflect.ClassTag
 import scala.util.Random
 
-import org.apache.spark.HashPartitioner
-import org.apache.spark.SparkContext._
 import org.apache.spark.SparkException
-import org.apache.spark.graphx.impl.EdgePartitionBuilder
-import org.apache.spark.graphx.impl.GraphImpl
 import org.apache.spark.graphx.lib._
 import org.apache.spark.rdd.RDD
 
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/VertexRDD.scala b/graphx/src/main/scala/org/apache/spark/graphx/VertexRDD.scala
index 5a0c479bb4..35577d9e2f 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/VertexRDD.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/VertexRDD.scala
@@ -20,7 +20,6 @@ package org.apache.spark.graphx
 import scala.reflect.ClassTag
 
 import org.apache.spark._
-import org.apache.spark.SparkContext._
 import org.apache.spark.graphx.impl.RoutingTablePartition
 import org.apache.spark.graphx.impl.ShippableVertexPartition
 import org.apache.spark.graphx.impl.VertexAttributeBlock
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/EdgePartition.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/EdgePartition.scala
index b1da781663..26349f4d88 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/EdgePartition.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/EdgePartition.scala
@@ -17,7 +17,7 @@
 
 package org.apache.spark.graphx.impl
 
-import scala.reflect.{classTag, ClassTag}
+import scala.reflect.ClassTag
 
 import org.apache.spark.graphx._
 import org.apache.spark.graphx.util.collection.GraphXPrimitiveKeyOpenHashMap
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/ReplicatedVertexView.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/ReplicatedVertexView.scala
index b4bec7cba5..d2194d85bf 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/ReplicatedVertexView.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/ReplicatedVertexView.scala
@@ -17,9 +17,8 @@
 
 package org.apache.spark.graphx.impl
 
-import scala.reflect.{classTag, ClassTag}
+import scala.reflect.ClassTag
 
-import org.apache.spark.SparkContext._
 import org.apache.spark.graphx._
 import org.apache.spark.rdd.RDD
 
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/RoutingTablePartition.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/RoutingTablePartition.scala
index 13e25b43f6..6453bbeae9 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/RoutingTablePartition.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/RoutingTablePartition.scala
@@ -17,14 +17,8 @@
 
 package org.apache.spark.graphx.impl
 
-import scala.reflect.ClassTag
-
-import org.apache.spark.Partitioner
 import org.apache.spark.graphx._
-import org.apache.spark.graphx.impl.RoutingTablePartition.RoutingTableMessage
 import org.apache.spark.graphx.util.collection.GraphXPrimitiveKeyOpenHashMap
-import org.apache.spark.rdd.RDD
-import org.apache.spark.rdd.ShuffledRDD
 import org.apache.spark.util.collection.{BitSet, PrimitiveVector}
 
 private[graphx]
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartition.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartition.scala
index 4512bc1739..b4100bade0 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartition.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexPartition.scala
@@ -20,7 +20,6 @@ package org.apache.spark.graphx.impl
 import scala.reflect.ClassTag
 
 import org.apache.spark.graphx._
-import org.apache.spark.graphx.util.collection.GraphXPrimitiveKeyOpenHashMap
 import org.apache.spark.util.collection.BitSet
 
 private[graphx] object VertexPartition {
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexRDDImpl.scala b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexRDDImpl.scala
index d5accdfbf7..d314522de9 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexRDDImpl.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/impl/VertexRDDImpl.scala
@@ -20,7 +20,6 @@ package org.apache.spark.graphx.impl
 import scala.reflect.ClassTag
 
 import org.apache.spark._
-import org.apache.spark.SparkContext._
 import org.apache.spark.graphx._
 import org.apache.spark.rdd._
 import org.apache.spark.storage.StorageLevel
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala b/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala
index 026fb8bc7b..34e9e22c3a 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/lib/TriangleCount.scala
@@ -20,7 +20,6 @@ package org.apache.spark.graphx.lib
 import scala.reflect.ClassTag
 
 import org.apache.spark.graphx._
-import org.apache.spark.graphx.PartitionStrategy.EdgePartition2D
 
 /**
  * Compute the number of triangles passing through each vertex.
diff --git a/graphx/src/main/scala/org/apache/spark/graphx/util/GraphGenerators.scala b/graphx/src/main/scala/org/apache/spark/graphx/util/GraphGenerators.scala
index 95522299f0..ba67c66f86 100644
--- a/graphx/src/main/scala/org/apache/spark/graphx/util/GraphGenerators.scala
+++ b/graphx/src/main/scala/org/apache/spark/graphx/util/GraphGenerators.scala
@@ -18,16 +18,12 @@
 package org.apache.spark.graphx.util
 
 import scala.annotation.tailrec
-import scala.math._
 import scala.reflect.ClassTag
 import scala.util._
 
 import org.apache.spark._
-import org.apache.spark.SparkContext._
 import org.apache.spark.graphx._
-import org.apache.spark.graphx.impl.GraphImpl
 import org.apache.spark.rdd.RDD
-import org.apache.spark.serializer._
 
 /** A collection of graph generating functions. */
 object GraphGenerators extends Logging {
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/GraphOpsSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/GraphOpsSuite.scala
index 3967f6683d..3298171949 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/GraphOpsSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/GraphOpsSuite.scala
@@ -19,8 +19,6 @@ package org.apache.spark.graphx
 
 import org.apache.spark.{SparkContext, SparkFunSuite}
 import org.apache.spark.graphx.Graph._
-import org.apache.spark.graphx.impl.EdgePartition
-import org.apache.spark.rdd._
 
 class GraphOpsSuite extends SparkFunSuite with LocalSparkContext {
 
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/PregelSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/PregelSuite.scala
index 8afa2d403b..90a9ac613e 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/PregelSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/PregelSuite.scala
@@ -17,8 +17,7 @@
 
 package org.apache.spark.graphx
 
-import org.apache.spark.{SparkContext, SparkFunSuite}
-import org.apache.spark.rdd._
+import org.apache.spark.SparkFunSuite
 
 class PregelSuite extends SparkFunSuite with LocalSparkContext {
 
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/impl/EdgePartitionSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/impl/EdgePartitionSuite.scala
index a73dfd219e..e4678b3578 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/impl/EdgePartitionSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/impl/EdgePartitionSuite.scala
@@ -18,7 +18,6 @@
 package org.apache.spark.graphx.impl
 
 import scala.reflect.ClassTag
-import scala.util.Random
 
 import org.apache.spark.{SparkConf, SparkFunSuite}
 import org.apache.spark.graphx._
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/lib/ConnectedComponentsSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/lib/ConnectedComponentsSuite.scala
index c965a6eb8d..1b81423563 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/lib/ConnectedComponentsSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/lib/ConnectedComponentsSuite.scala
@@ -17,8 +17,7 @@
 
 package org.apache.spark.graphx.lib
 
-import org.apache.spark.{SparkContext, SparkFunSuite}
-import org.apache.spark.SparkContext._
+import org.apache.spark.SparkFunSuite
 import org.apache.spark.graphx._
 import org.apache.spark.graphx.util.GraphGenerators
 import org.apache.spark.rdd._
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala
index d7eaa70ce6..994395bbff 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/lib/ShortestPathsSuite.scala
@@ -17,12 +17,8 @@
 
 package org.apache.spark.graphx.lib
 
-import org.apache.spark.{SparkContext, SparkFunSuite}
-import org.apache.spark.SparkContext._
+import org.apache.spark.SparkFunSuite
 import org.apache.spark.graphx._
-import org.apache.spark.graphx.lib._
-import org.apache.spark.graphx.util.GraphGenerators
-import org.apache.spark.rdd._
 
 class ShortestPathsSuite extends SparkFunSuite with LocalSparkContext {
 
diff --git a/graphx/src/test/scala/org/apache/spark/graphx/lib/StronglyConnectedComponentsSuite.scala b/graphx/src/test/scala/org/apache/spark/graphx/lib/StronglyConnectedComponentsSuite.scala
index d6b0320818..2c57e8927e 100644
--- a/graphx/src/test/scala/org/apache/spark/graphx/lib/StronglyConnectedComponentsSuite.scala
+++ b/graphx/src/test/scala/org/apache/spark/graphx/lib/StronglyConnectedComponentsSuite.scala
@@ -17,11 +17,8 @@
 
 package org.apache.spark.graphx.lib
 
-import org.apache.spark.{SparkContext, SparkFunSuite}
-import org.apache.spark.SparkContext._
+import org.apache.spark.SparkFunSuite
 import org.apache.spark.graphx._
-import org.apache.spark.graphx.util.GraphGenerators
-import org.apache.spark.rdd._
 
 
 class StronglyConnectedComponentsSuite extends SparkFunSuite with LocalSparkContext {