code: public final ListTopicSubscriptionsPagedResponse listTopicSubscriptions(ProjectTopicName topic) {
      ListTopicSubscriptionsRequest request =
          ListTopicSubscriptionsRequest.newBuilder()
              .setTopic(topic == null ? null : topic.toString())
              .build();
      return listTopicSubscriptions(request);
    }
docs: Lists the names of the subscriptions on this topic.
  
  <p>Sample code:
  
  <pre><code>
  try (TopicAdminClient topicAdminClient = TopicAdminClient.create()) {
  ProjectTopicName topic = ProjectTopicName.of("[PROJECT]", "[TOPIC]");
  for (ProjectSubscriptionName element : topicAdminClient.listTopicSubscriptions(topic).iterateAllAsProjectSubscriptionName()) {
  // doThingsWith(element);
  }
  }
  </code></pre>
  
  @param topic The name of the topic that subscriptions are attached to. Format is
  `projects/{project}/topics/{topic}`.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails