Enforce overrinding execute() method in IntentUpdate subclasses

Change-Id: I02cf03abd60f771357b98bbd79c83eb3e0dce958

@@ -18,8 +18,6 @@ package org.onosproject.net.intent.impl;
 import org.onosproject.net.intent.BatchWrite;
 import org.onosproject.net.intent.Intent;
 
-import java.util.Optional;
-
 import static com.google.common.base.Preconditions.checkNotNull;
 import static org.onosproject.net.intent.IntentState.FAILED;
 
@@ -40,11 +38,6 @@ class CompilingFailed implements CompletedIntentUpdate {
     }
 
     @Override
-    public Optional<IntentUpdate> execute() {
-        return Optional.empty();
-    }
-
-    @Override
     public void writeAfterExecution(BatchWrite batchWrite) {
         batchWrite.setState(intent, FAILED);
         batchWrite.removeInstalledIntents(intent.id());

@@ -21,6 +21,7 @@ import org.onosproject.net.intent.Intent;
 
 import java.util.Collections;
 import java.util.List;
+import java.util.Optional;
 
 /**
  * Represents a completed phase of processing an intent.
@@ -63,4 +64,9 @@ interface CompletedIntentUpdate extends IntentUpdate {
     default List<Intent> allInstallables() {
         return Collections.emptyList();
     }
+
+    @Override
+    default Optional<IntentUpdate> execute() {
+        return Optional.empty();
+    }
 }

@@ -30,9 +30,7 @@ interface IntentUpdate {
      *
      * @return next update
      */
-    default Optional<IntentUpdate> execute() {
-        return Optional.empty();
-    }
+    Optional<IntentUpdate> execute();
 
     /**
      * Write data to the specified BatchWrite before execution() is called.