[hotfix] [core] Fix checkstyle in flink-core:'org.apache.flink.api.common.io.compression'

flink-core/src/main/java/org/apache/flink/api/common/io/compression/Bzip2InputStreamFactory.java

@@ -15,26 +15,27 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.apache.flink.api.common.io.compression;
 
 import org.apache.flink.annotation.Internal;
 
+import org.apache.commons.compress.compressors.bzip2.BZip2CompressorInputStream;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.Collection;
 import java.util.Collections;
 
-import org.apache.commons.compress.compressors.bzip2.BZip2CompressorInputStream;
-
+/**
+ * Factory for Bzip2 decompressors.
+ */
 @Internal
 public class Bzip2InputStreamFactory implements InflaterInputStreamFactory<BZip2CompressorInputStream> {
 
-	private static Bzip2InputStreamFactory INSTANCE = null;
+	private static final Bzip2InputStreamFactory INSTANCE = new Bzip2InputStreamFactory();
 
 	public static Bzip2InputStreamFactory getInstance() {
-		if (INSTANCE == null) {
-			INSTANCE = new Bzip2InputStreamFactory();
-		}
 		return INSTANCE;
 	}
 

flink-core/src/main/java/org/apache/flink/api/common/io/compression/DeflateInflaterInputStreamFactory.java

@@ -15,6 +15,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.apache.flink.api.common.io.compression;
 
 import org.apache.flink.annotation.Internal;
@@ -31,12 +32,9 @@ import java.util.zip.InflaterInputStream;
 @Internal
 public class DeflateInflaterInputStreamFactory implements InflaterInputStreamFactory<InflaterInputStream> {
 
-	private static DeflateInflaterInputStreamFactory INSTANCE = null;
+	private static final DeflateInflaterInputStreamFactory INSTANCE = new DeflateInflaterInputStreamFactory();
 
 	public static DeflateInflaterInputStreamFactory getInstance() {
-		if (INSTANCE == null) {
-			INSTANCE = new DeflateInflaterInputStreamFactory();
-		}
 		return INSTANCE;
 	}
 

flink-core/src/main/java/org/apache/flink/api/common/io/compression/GzipInflaterInputStreamFactory.java

@@ -15,6 +15,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.apache.flink.api.common.io.compression;
 
 import org.apache.flink.annotation.Internal;
@@ -31,14 +32,12 @@ import java.util.zip.GZIPInputStream;
 @Internal
 public class GzipInflaterInputStreamFactory implements InflaterInputStreamFactory<GZIPInputStream> {
 
-	private static GzipInflaterInputStreamFactory INSTANCE = null;
+	private static final GzipInflaterInputStreamFactory INSTANCE = new GzipInflaterInputStreamFactory();
 
 	public static GzipInflaterInputStreamFactory getInstance() {
-		if (INSTANCE == null) {
-			INSTANCE = new GzipInflaterInputStreamFactory();
-		}
 		return INSTANCE;
 	}
+
 	@Override
 	public GZIPInputStream create(InputStream in) throws IOException {
 		return new GZIPInputStream(in);

flink-core/src/main/java/org/apache/flink/api/common/io/compression/XZInputStreamFactory.java

@@ -15,25 +15,27 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.apache.flink.api.common.io.compression;
 
-import org.apache.commons.compress.compressors.xz.XZCompressorInputStream;
 import org.apache.flink.annotation.Internal;
 
+import org.apache.commons.compress.compressors.xz.XZCompressorInputStream;
+
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.Collection;
 import java.util.Collections;
 
+/**
+ * Factory for XZ decompressors.
+ */
 @Internal
 public class XZInputStreamFactory implements InflaterInputStreamFactory<XZCompressorInputStream> {
 
-	private static XZInputStreamFactory INSTANCE = null;
+	private static final  XZInputStreamFactory INSTANCE = new XZInputStreamFactory();
 
 	public static XZInputStreamFactory getInstance() {
-		if (INSTANCE == null) {
-			INSTANCE = new XZInputStreamFactory();
-		}
 		return INSTANCE;
 	}