in flink-connector-gcp-pubsub/src/main/java/org/apache/flink/streaming/connectors/gcp/pubsub/PubSubSink.java [274:278]
public PubSubSinkBuilder<IN> withTopicName(String topicName) {
Preconditions.checkNotNull(topicName);
this.topicName = topicName;
return this;
}