Hi, I am Implementing user defined aggregate function and using the bellow Accumulator class, but getting bellow exception. the exception says Set is abstract and it is not allowed but

List

works fine

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class Accumulator {
    private double sum = 0;
    private long count = 0;
    private double minValue = Double.MAX_VALUE;
    private double maxValue = Double.MIN_VALUE;
    private List<Double> values = new ArrayList<>();
    private Set<String> buffer = new HashSet<>();
}

Caused by: org.apache.flink.table.api.ValidationException: Could not extract a data type from 'class org.abs.security.utils.Accumulator' in generic class 'org.apache.flink.table.functions.AggregateFunction' in class org.abs.security.udfs.DistinctCount. Please pass the required data type manually or allow RAW types.
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:362)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrRawWithTemplate(DataTypeExtractor.java:240)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrRaw(DataTypeExtractor.java:218)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeWithClassContext(DataTypeExtractor.java:194)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractFromGeneric(DataTypeExtractor.java:124)
	at org.apache.flink.table.types.extraction.FunctionMappingExtractor.lambda$createGenericResultExtraction$13(FunctionMappingExtractor.java:478)
	at org.apache.flink.table.types.extraction.FunctionMappingExtractor.putExtractedResultMappings(FunctionMappingExtractor.java:319)
	at org.apache.flink.table.types.extraction.FunctionMappingExtractor.collectMethodMappings(FunctionMappingExtractor.java:269)
	at org.apache.flink.table.types.extraction.FunctionMappingExtractor.extractResultMappings(FunctionMappingExtractor.java:169)
	... 23 more
Caused by: org.apache.flink.table.api.ValidationException: Could not extract a data type from 'class org.abs.security.utils.Accumulator'. Interpreting it as a structured type was also not successful.
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:362)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrError(DataTypeExtractor.java:290)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrRawWithTemplate(DataTypeExtractor.java:232)
	... 30 more
Caused by: org.apache.flink.table.api.ValidationException: Error in field 'buffer' of class 'org.abs.security.utils.Accumulator'.
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:362)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractStructuredTypeFields(DataTypeExtractor.java:539)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractStructuredType(DataTypeExtractor.java:513)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrError(DataTypeExtractor.java:288)
	... 31 more
Caused by: org.apache.flink.table.api.ValidationException: Could not extract a data type from 'java.util.Set<java.lang.String>' in generic class 'org.apache.flink.table.functions.AggregateFunction' in class org.abs.security.udfs.DistinctCount. Please pass the required data type manually or allow RAW types.
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:362)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrRawWithTemplate(DataTypeExtractor.java:240)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrRaw(DataTypeExtractor.java:218)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractStructuredTypeFields(DataTypeExtractor.java:536)
	... 33 more
Caused by: org.apache.flink.table.api.ValidationException: Could not extract a data type from 'java.util.Set<java.lang.String>'. Interpreting it as a structured type was also not successful.
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:362)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrError(DataTypeExtractor.java:290)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrRawWithTemplate(DataTypeExtractor.java:232)
	... 35 more
Caused by: org.apache.flink.table.api.ValidationException: Class 'java.util.Set' must not be abstract.
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:362)
	at org.apache.flink.table.types.extraction.ExtractionUtils.extractionError(ExtractionUtils.java:357)
	at org.apache.flink.table.types.extraction.ExtractionUtils.validateStructuredClass(ExtractionUtils.java:165)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractStructuredType(DataTypeExtractor.java:478)
	at org.apache.flink.table.types.extraction.DataTypeExtractor.extractDataTypeOrError(DataTypeExtractor.java:288)
	... 36 more

Currently, Flink's type inference does not support auto mapping

Set

to MULTISET (See https://nightlies.apache.org/flink/flink-docs-release-1.17/docs/dev/table/types/#data-type-extraction). It's recommended to use

DataTypeHint

as a workaround. (https://nightlies.apache.org/flink/flink-docs-release-1.17/docs/dev/table/functions/udfs/#type-inference)