in flink-connector-gcp-pubsub/src/main/java/org/apache/flink/streaming/connectors/gcp/pubsub/common/AcknowledgeOnCheckpoint.java [58:63]
public AcknowledgeOnCheckpoint(Acknowledger<ACKID> acknowledger) {
this.acknowledger = acknowledger;
this.acknowledgeIdsPerCheckpoint = new ArrayList<>();
this.acknowledgeIdsForPendingCheckpoint = new ArrayList<>();
this.outstandingAcknowledgements = new AtomicInteger(0);
}