HDFS-16027. Replace abstract methods with default methods in JournalNodeMXBean. (#3018)

Reviewed-by: Ayush Saxena <ayushsaxena@apache.org>

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/qjournal/server/JournalNodeMXBean.java

@@ -20,6 +20,7 @@ package org.apache.hadoop.hdfs.qjournal.server;
 import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.classification.InterfaceStability;
 
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -41,7 +42,9 @@ public interface JournalNodeMXBean {
    *
    * @return colon separated host and port.
    */
-  String getHostAndPort();
+  default String getHostAndPort() {
+    return "";
+  }
 
   /**
    * Get list of the clusters of JournalNode's journals
@@ -49,12 +52,16 @@ public interface JournalNodeMXBean {
    *
    * @return list of clusters.
    */
-  List<String> getClusterIds();
+  default List<String> getClusterIds() {
+    return Collections.emptyList() ;
+  }
 
   /**
    * Gets the version of Hadoop.
    *
    * @return the version of Hadoop.
    */
-  String getVersion();
+  default String getVersion() {
+    return "";
+  }
 }