Added usage of ReadEventHandler as functional interface

topic/src/main/java/tech/ydb/topic/read/events/AbstractReadEventHandler.java

@@ -4,31 +4,4 @@
  * @author Nikolay Perfilov
  */
 public abstract class AbstractReadEventHandler implements ReadEventHandler {
-
-    // onMessages(DataReceivedEvent event) method should be defined in user's implementation
-
-    @Override
-    public void onCommitResponse(CommitOffsetAcknowledgementEvent event) {
-
-    }
-
-    @Override
-    public void onStartPartitionSession(StartPartitionSessionEvent event) {
-        event.confirm();
-    }
-
-    @Override
-    public void onStopPartitionSession(StopPartitionSessionEvent event) {
-        event.confirm();
-    }
-
-    @Override
-    public void onPartitionSessionClosed(PartitionSessionClosedEvent event) {
-
-    }
-
-    @Override
-    public void onReaderClosed(ReaderClosedEvent event) {
-
-    }
 }

topic/src/main/java/tech/ydb/topic/read/events/ReadEventHandler.java

@@ -3,17 +3,23 @@
 /**
  * @author Nikolay Perfilov
  */
+@FunctionalInterface
 public interface ReadEventHandler {
 
     void onMessages(DataReceivedEvent event);
 
-    void onCommitResponse(CommitOffsetAcknowledgementEvent event);
+    default void onCommitResponse(CommitOffsetAcknowledgementEvent event) { }
 
-    void onStartPartitionSession(StartPartitionSessionEvent event);
 
-    void onStopPartitionSession(StopPartitionSessionEvent event);
+    default void onStartPartitionSession(StartPartitionSessionEvent event) {
+        event.confirm();
+    }
 
-    void onPartitionSessionClosed(PartitionSessionClosedEvent event);
+    default void onStopPartitionSession(StopPartitionSessionEvent event) {
+        event.confirm();
+    }
 
-    void onReaderClosed(ReaderClosedEvent event);
+    default void onPartitionSessionClosed(PartitionSessionClosedEvent event) { }
+
+    default void onReaderClosed(ReaderClosedEvent event) { }
 }