private com.google.protobuf.SingleFieldBuilderV3<
            com.google.apps.events.subscriptions.v1.NotificationEndpoint,
            com.google.apps.events.subscriptions.v1.NotificationEndpoint.Builder,
            com.google.apps.events.subscriptions.v1.NotificationEndpointOrBuilder>
        getNotificationEndpointFieldBuilder() {
      if (notificationEndpointBuilder_ == null) {
        notificationEndpointBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.apps.events.subscriptions.v1.NotificationEndpoint,
                com.google.apps.events.subscriptions.v1.NotificationEndpoint.Builder,
                com.google.apps.events.subscriptions.v1.NotificationEndpointOrBuilder>(
                getNotificationEndpoint(), getParentForChildren(), isClean());
        notificationEndpoint_ = null;
      }
      return notificationEndpointBuilder_;
    }
<pre>
Required. Immutable. The endpoint where the subscription delivers events,
such as a Pub/Sub topic.
</pre>

<code>
.google.apps.events.subscriptions.v1.NotificationEndpoint notification_endpoint = 7 [(.google.api.field_behavior) = REQUIRED, (.google.api.field_behavior) = IMMUTABLE];
</code>
java-workspaceevents/proto-google-cloud-workspaceevents-v1/src/main/java/com/google/apps/events/subscriptions/v1/Subscription.java