[SPARK-18420][BUILD] Fix the errors caused by lint check in Java

## What changes were proposed in this pull request?

Small fix, fix the errors caused by lint check in Java

- Clear unused objects and `UnusedImports`.
- Add comments around the method `finalize` of `NioBufferedFileInputStream`to turn off checkstyle.
- Cut the line which is longer than 100 characters into two lines.

## How was this patch tested?
Travis CI.
```
$ build/mvn -T 4 -q -DskipTests -Pyarn -Phadoop-2.3 -Pkinesis-asl -Phive -Phive-thriftserver install
$ dev/lint-java
```
Before:
```
Checkstyle checks failed at following occurrences:
[ERROR] src/main/java/org/apache/spark/network/util/TransportConf.java:[21,8] (imports) UnusedImports: Unused import - org.apache.commons.crypto.cipher.CryptoCipherFactory.
[ERROR] src/test/java/org/apache/spark/network/sasl/SparkSaslSuite.java:[516,5] (modifier) RedundantModifier: Redundant 'public' modifier.
[ERROR] src/main/java/org/apache/spark/io/NioBufferedFileInputStream.java:[133] (coding) NoFinalizer: Avoid using finalizer method.
[ERROR] src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeMapData.java:[71] (sizes) LineLength: Line is longer than 100 characters (found 113).
[ERROR] src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeArrayData.java:[112] (sizes) LineLength: Line is longer than 100 characters (found 110).
[ERROR] src/test/java/org/apache/spark/sql/catalyst/expressions/HiveHasherSuite.java:[31,17] (modifier) ModifierOrder: 'static' modifier out of order with the JLS suggestions.
[ERROR]src/main/java/org/apache/spark/examples/ml/JavaLogisticRegressionWithElasticNetExample.java:[64] (sizes) LineLength: Line is longer than 100 characters (found 103).
[ERROR] src/main/java/org/apache/spark/examples/ml/JavaInteractionExample.java:[22,8] (imports) UnusedImports: Unused import - org.apache.spark.ml.linalg.Vectors.
[ERROR] src/main/java/org/apache/spark/examples/ml/JavaInteractionExample.java:[51] (regexp) RegexpSingleline: No trailing whitespace allowed.
```

After:
```
$ build/mvn -T 4 -q -DskipTests -Pyarn -Phadoop-2.3 -Pkinesis-asl -Phive -Phive-thriftserver install
$ dev/lint-java
Using `mvn` from path: /home/travis/build/ConeyLiu/spark/build/apache-maven-3.3.9/bin/mvn
Checkstyle checks passed.
```

Author: Xianyang Liu <xyliu0530@icloud.com>

Closes #15865 from ConeyLiu/master.

Author: Xianyang Liu

common/network-common/src/main/java/org/apache/spark/network/util/TransportConf.java

@@ -18,7 +18,6 @@
 package org.apache.spark.network.util;
 
 import com.google.common.primitives.Ints;
-import org.apache.commons.crypto.cipher.CryptoCipherFactory;
 
 /**
  * A central location that tracks all the settings we expose to users.

common/network-common/src/test/java/org/apache/spark/network/sasl/SparkSaslSuite.java

@@ -513,7 +513,7 @@ private static class EncryptionCheckerBootstrap extends ChannelOutboundHandlerAd
     boolean foundEncryptionHandler;
     String encryptHandlerName;
 
-    public EncryptionCheckerBootstrap(String encryptHandlerName) {
+    EncryptionCheckerBootstrap(String encryptHandlerName) {
       this.encryptHandlerName = encryptHandlerName;
     }
 

core/src/main/java/org/apache/spark/io/NioBufferedFileInputStream.java

@@ -130,8 +130,10 @@ public synchronized void close() throws IOException {
     StorageUtils.dispose(byteBuffer);
   }
 
+  //checkstyle.off: NoFinalizer
   @Override
   protected void finalize() throws IOException {
     close();
   }
+  //checkstyle.on: NoFinalizer
 }

dev/checkstyle.xml

@@ -52,6 +52,20 @@
       <property name="file" value="dev/checkstyle-suppressions.xml"/>
     </module>
 
+    <!--
+    If you wish to turn off checking for a section of code, you can put a comment in the source
+    before and after the section, with the following syntax:
+
+      // checkstyle:off no.XXX (such as checkstyle.off: NoFinalizer)
+      ...  // stuff that breaks the styles
+      // checkstyle:on
+    -->
+    <module name="SuppressionCommentFilter">
+        <property name="offCommentFormat" value="checkstyle.off\: ([\w\|]+)"/>
+        <property name="onCommentFormat" value="checkstyle.on\: ([\w\|]+)"/>
+        <property name="checkFormat" value="$1"/>
+    </module>
+
     <!-- Checks for whitespace                               -->
     <!-- See http://checkstyle.sf.net/config_whitespace.html -->
     <module name="FileTabCharacter">
@@ -168,5 +182,6 @@
         <module name="UnusedImports"/>
         <module name="RedundantImport"/>
         <module name="RedundantModifier"/>
+        <module name="FileContentsHolder"/>
     </module>
 </module>

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

@@ -19,7 +19,6 @@
 
 import org.apache.spark.ml.feature.Interaction;
 import org.apache.spark.ml.feature.VectorAssembler;
-import org.apache.spark.ml.linalg.Vectors;
 import org.apache.spark.sql.*;
 import org.apache.spark.sql.types.DataTypes;
 import org.apache.spark.sql.types.Metadata;
@@ -48,7 +47,7 @@ public static void main(String[] args) {
       RowFactory.create(5, 9, 2, 7, 10, 7, 3),
       RowFactory.create(6, 1, 1, 4, 2, 8, 4)
     );
-    
+
     StructType schema = new StructType(new StructField[]{
       new StructField("id1", DataTypes.IntegerType, false, Metadata.empty()),
       new StructField("id2", DataTypes.IntegerType, false, Metadata.empty()),

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

@@ -60,8 +60,8 @@ public static void main(String[] args) {
     LogisticRegressionModel mlrModel = mlr.fit(training);
 
     // Print the coefficients and intercepts for logistic regression with multinomial family
-    System.out.println("Multinomial coefficients: "
-            + lrModel.coefficientMatrix() + "\nMultinomial intercepts: " + mlrModel.interceptVector());
+    System.out.println("Multinomial coefficients: " + lrModel.coefficientMatrix()
+      + "\nMultinomial intercepts: " + mlrModel.interceptVector());
     // $example off$
 
     spark.stop();

sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeArrayData.java

@@ -109,7 +109,8 @@ public void pointTo(Object baseObject, long baseOffset, int sizeInBytes) {
     // Read the number of elements from the first 8 bytes.
     final long numElements = Platform.getLong(baseObject, baseOffset);
     assert numElements >= 0 : "numElements (" + numElements + ") should >= 0";
-    assert numElements <= Integer.MAX_VALUE : "numElements (" + numElements + ") should <= Integer.MAX_VALUE";
+    assert numElements <= Integer.MAX_VALUE :
+      "numElements (" + numElements + ") should <= Integer.MAX_VALUE";
 
     this.numElements = (int)numElements;
     this.baseObject = baseObject;

sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/UnsafeMapData.java

@@ -68,7 +68,8 @@ public void pointTo(Object baseObject, long baseOffset, int sizeInBytes) {
     // Read the numBytes of key array from the first 8 bytes.
     final long keyArraySize = Platform.getLong(baseObject, baseOffset);
     assert keyArraySize >= 0 : "keyArraySize (" + keyArraySize + ") should >= 0";
-    assert keyArraySize <= Integer.MAX_VALUE : "keyArraySize (" + keyArraySize + ") should <= Integer.MAX_VALUE";
+    assert keyArraySize <= Integer.MAX_VALUE :
+      "keyArraySize (" + keyArraySize + ") should <= Integer.MAX_VALUE";
     final int valueArraySize = sizeInBytes - (int)keyArraySize - 8;
     assert valueArraySize >= 0 : "valueArraySize (" + valueArraySize + ") should >= 0";
 

sql/catalyst/src/test/java/org/apache/spark/sql/catalyst/expressions/HiveHasherSuite.java

@@ -28,7 +28,6 @@
 import java.util.Set;
 
 public class HiveHasherSuite {
-  private final static HiveHasher hasher = new HiveHasher();
 
   @Test
   public void testKnownIntegerInputs() {