samples: exit early if no messages are returned (#989)

* samples: exit early if no messages are pulled * address kurtis's comments * update comment

Author: anguillanneuf

samples/snippets/src/main/java/pubsub/SubscribeSyncExample.java

@@ -60,12 +60,21 @@ public static void subscribeSyncExample(
 
  // Use pullCallable().futureCall to asynchronously perform this operation.
  PullResponse pullResponse = subscriber.pullCallable().call(pullRequest);
+
+ // Stop the program if the pull response is empty to avoid acknowledging
+ // an empty list of ack IDs.
+ if (pullResponse.getReceivedMessagesList().isEmpty()) {
+ System.out.println("No message was pulled. Exiting.");
+ return;
+ }
+
  List<String> ackIds = new ArrayList<>();
  for (ReceivedMessage message : pullResponse.getReceivedMessagesList()) {
  // Handle received message
  // ...
  ackIds.add(message.getAckId());
  }
+
  // Acknowledge received messages.
  AcknowledgeRequest acknowledgeRequest =
  AcknowledgeRequest.newBuilder()

samples/snippets/src/main/java/pubsub/SubscribeSyncWithLeaseExample.java

@@ -38,9 +38,6 @@ public static void main(String... args) throws Exception {
  String subscriptionId = "your-subscription-id";
  Integer numOfMessages = 10;
 
- projectId = "tz-playground-bigdata";
- subscriptionId = "uno";
-
  subscribeSyncWithLeaseExample(projectId, subscriptionId, numOfMessages);
  }
 
@@ -68,8 +65,14 @@ public static void subscribeSyncWithLeaseExample(
  // Use pullCallable().futureCall to asynchronously perform this operation.
  PullResponse pullResponse = subscriber.pullCallable().call(pullRequest);
 
- List<String> ackIds = new ArrayList<>();
+ // Stop the program if the pull response is empty to avoid acknowledging
+ // an empty list of ack IDs.
+ if (pullResponse.getReceivedMessagesList().isEmpty()) {
+ System.out.println("No message was pulled. Exiting.");
+ return;
+ }
 
+ List<String> ackIds = new ArrayList<>();
  for (ReceivedMessage message : pullResponse.getReceivedMessagesList()) {
  ackIds.add(message.getAckId());