[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 apache#18345 from dongjoon-hyun/fix_lint_java_2.

common/kvstore/src/main/java/org/apache/spark/kvstore/KVIndex.java

@@ -50,7 +50,7 @@
 @Target({ElementType.FIELD, ElementType.METHOD})
 public @interface KVIndex {
 
-  public static final String NATURAL_INDEX_NAME = "__main__";
+  String NATURAL_INDEX_NAME = "__main__";
 
   /**
    * The name of the index to be created for the annotated entity. Must be unique within

common/kvstore/src/main/java/org/apache/spark/kvstore/KVStore.java

@@ -18,9 +18,6 @@
 package org.apache.spark.kvstore;
 
 import java.io.Closeable;
-import java.util.Iterator;
-import java.util.Map;
-import java.util.NoSuchElementException;
 
 /**
  * Abstraction for a local key/value store for storing app data.
@@ -84,7 +81,7 @@ public interface KVStore extends Closeable {
    *
    * @param naturalKey The object's "natural key", which uniquely identifies it. Null keys
    *                   are not allowed.
-   * @throws NoSuchElementException If an element with the given key does not exist.
+   * @throws java.util.NoSuchElementException If an element with the given key does not exist.
    */
   <T> T read(Class<T> klass, Object naturalKey) throws Exception;
 
@@ -107,7 +104,7 @@ public interface KVStore extends Closeable {
    * @param type The object's type.
    * @param naturalKey The object's "natural key", which uniquely identifies it. Null keys
    *                   are not allowed.
-   * @throws NoSuchElementException If an element with the given key does not exist.
+   * @throws java.util.NoSuchElementException If an element with the given key does not exist.
    */
   void delete(Class<?> type, Object naturalKey) throws Exception;
 

common/kvstore/src/main/java/org/apache/spark/kvstore/KVStoreView.java

@@ -17,9 +17,6 @@
 
 package org.apache.spark.kvstore;
 
-import java.util.Iterator;
-import java.util.Map;
-
 import com.google.common.base.Preconditions;
 
 /**

common/kvstore/src/main/java/org/apache/spark/kvstore/KVTypeInfo.java

@@ -19,8 +19,6 @@
 
 import java.lang.reflect.Field;
 import java.lang.reflect.Method;
-import java.util.ArrayList;
-import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.stream.Stream;

common/kvstore/src/main/java/org/apache/spark/kvstore/LevelDB.java

@@ -29,7 +29,6 @@
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.common.annotations.VisibleForTesting;
-import com.google.common.base.Objects;
 import com.google.common.base.Preconditions;
 import com.google.common.base.Throwables;
 import org.fusesource.leveldbjni.JniDBFactory;

common/kvstore/src/main/java/org/apache/spark/kvstore/LevelDBIterator.java

@@ -18,7 +18,6 @@
 package org.apache.spark.kvstore;
 
 import java.io.IOException;
-import java.util.Arrays;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;

common/kvstore/src/main/java/org/apache/spark/kvstore/LevelDBTypeInfo.java

@@ -18,17 +18,12 @@
 package org.apache.spark.kvstore;
 
 import java.lang.reflect.Array;
-import java.lang.reflect.Field;
-import java.lang.reflect.Method;
-import java.io.ByteArrayOutputStream;
-import java.io.IOException;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Map;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.common.base.Preconditions;
-import com.google.common.base.Throwables;
 import org.iq80.leveldb.WriteBatch;
 
 /**

common/kvstore/src/test/java/org/apache/spark/kvstore/DBIteratorSuite.java

@@ -25,11 +25,9 @@
 import java.util.List;
 import java.util.Random;
 
-import com.google.common.base.Predicate;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Iterators;
 import com.google.common.collect.Lists;
-import org.apache.commons.io.FileUtils;
 import org.junit.AfterClass;
 import org.junit.Before;
 import org.junit.BeforeClass;
@@ -50,7 +48,7 @@ public abstract class DBIteratorSuite {
   private static List<CustomType1> clashingEntries;
   private static KVStore db;
 
-  private static interface BaseComparator extends Comparator<CustomType1> {
+  private interface BaseComparator extends Comparator<CustomType1> {
     /**
      * Returns a comparator that falls back to natural order if this comparator's ordering
      * returns equality for two elements. Used to mimic how the index sorts things internally.

common/kvstore/src/test/java/org/apache/spark/kvstore/LevelDBSuite.java

@@ -20,9 +20,7 @@
 import java.io.File;
 import java.util.Arrays;
 import java.util.List;
-import java.util.Map;
 import java.util.NoSuchElementException;
-import static java.nio.charset.StandardCharsets.UTF_8;
 
 import org.apache.commons.io.FileUtils;
 import org.iq80.leveldb.DBIterator;

common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/OneForOneBlockFetcher.java

@@ -157,7 +157,7 @@ private class DownloadCallback implements StreamCallback {
     private File targetFile = null;
     private int chunkIndex;
 
-    public DownloadCallback(File targetFile, int chunkIndex) throws IOException {
+    DownloadCallback(File targetFile, int chunkIndex) throws IOException {
       this.targetFile = targetFile;
       this.channel = Channels.newChannel(new FileOutputStream(targetFile));
       this.chunkIndex = chunkIndex;

core/src/main/java/org/apache/spark/shuffle/sort/UnsafeShuffleWriter.java

@@ -364,7 +364,8 @@ private long[] mergeSpillsWithFileStream(
     // Use a counting output stream to avoid having to close the underlying file and ask
     // the file system for its size after each partition is written.
     final CountingOutputStream mergedFileOutputStream = new CountingOutputStream(bos);
-    final int inputBufferSizeInBytes = (int) sparkConf.getSizeAsKb("spark.shuffle.file.buffer", "32k") * 1024;
+    final int inputBufferSizeInBytes =
+      (int) sparkConf.getSizeAsKb("spark.shuffle.file.buffer", "32k") * 1024;
 
     boolean threwException = true;
     try {
@@ -375,8 +376,9 @@ private long[] mergeSpillsWithFileStream(
       }
       for (int partition = 0; partition < numPartitions; partition++) {
         final long initialFileLength = mergedFileOutputStream.getByteCount();
-        // Shield the underlying output stream from close() and flush() calls, so that we can close the higher
-        // level streams to make sure all data is really flushed and internal state is cleaned.
+        // Shield the underlying output stream from close() and flush() calls, so that we can close
+        // the higher level streams to make sure all data is really flushed and internal state is
+        // cleaned.
         OutputStream partitionOutput = new CloseAndFlushShieldOutputStream(
           new TimeTrackingOutputStream(writeMetrics, mergedFileOutputStream));
         partitionOutput = blockManager.serializerManager().wrapForEncryption(partitionOutput);

examples/src/main/java/org/apache/spark/examples/ml/JavaALSExample.java

@@ -121,7 +121,7 @@ public static void main(String[] args) {
     // $example off$
     userRecs.show();
     movieRecs.show();
-    
+
     spark.stop();
   }
 }

examples/src/main/java/org/apache/spark/examples/sql/JavaSQLDataSourceExample.java

@@ -124,7 +124,11 @@ private static void runBasicDataSourceExample(SparkSession spark) {
     peopleDF.write().bucketBy(42, "name").sortBy("age").saveAsTable("people_bucketed");
     // $example off:write_sorting_and_bucketing$
     // $example on:write_partitioning$
-    usersDF.write().partitionBy("favorite_color").format("parquet").save("namesPartByColor.parquet");
+    usersDF
+      .write()
+      .partitionBy("favorite_color")
+      .format("parquet")
+      .save("namesPartByColor.parquet");
     // $example off:write_partitioning$
     // $example on:write_partition_and_bucket$
     peopleDF

sql/catalyst/src/main/java/org/apache/spark/sql/streaming/OutputMode.java

@@ -17,7 +17,6 @@
 
 package org.apache.spark.sql.streaming;
 
-import org.apache.spark.annotation.Experimental;
 import org.apache.spark.annotation.InterfaceStability;
 import org.apache.spark.sql.catalyst.streaming.InternalOutputModes;