public void setPartitionNumber(String partitionNumber) {
        // partition Number is optional parameter and not always needed, so it can be null.
        if (partitionNumber != null) {
            this.partitionNumber = Integer.parseInt(partitionNumber);
        }
    }
output: integer list of partitionKeys' corresponding positions within schema
inlong-sort/sort-flink/sort-flink-v1.13/sort-connectors/kafka/src/main/java/org/apache/inlong/sort/kafka/partitioner/SingleTableCustomFieldsPartitioner.java