in flink-connector-gcp-pubsub/src/main/java/org/apache/flink/streaming/connectors/gcp/pubsub/PubSubSource.java [268:272]
public PubSubSourceBuilder<OUT> withSubscriptionName(String subscriptionName) {
Preconditions.checkNotNull(subscriptionName);
this.subscriptionName = subscriptionName;
return this;
}