{"record":{"id":"63f250e8b1222352","repo":"apache/flink","slug":"reached-the-end-of-the-collection-this-could-be-c-63f250","errorCode":null,"errorMessage":"Reached the end of the collection. This could be caused by issues with the serializer or by calling the map() function more times than there are elements in the collection. Make sure that you set the number of records to be produced by the DataGeneratorSource equal to the number of elements in the collection.","messagePattern":"Reached the end of the collection\\. This could be caused by issues with the serializer or by calling the map\\(\\) function more times than there are elements in the collection\\. Make sure that you set the number of records to be produced by the DataGeneratorSource equal to the number of elements in the collection\\.","errorType":"exception","errorClass":"NoSuchElementException","httpStatus":null,"severity":"error","filePath":"flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java","lineNumber":147,"sourceCode":"        LOG.info(\"Serializing elements using  {}\", serializer);\n        ByteArrayOutputStream baos = new ByteArrayOutputStream();\n        DataOutputViewStreamWrapper wrapper = new DataOutputViewStreamWrapper(baos);\n\n        try {\n            for (OUT element : elements) {\n                serializer.serialize(element, wrapper);\n            }\n        } catch (Exception e) {\n            throw new IOException(\"Serializing the source elements failed: \" + e.getMessage(), e);\n        }\n        this.elementsSerialized = baos.toByteArray();\n    }\n\n    private OUT tryDeserialize() throws IOException {\n        try {\n            return serializer.deserialize(input);\n        } catch (EOFException eof) {\n            throw new NoSuchElementException(\n                    \"Reached the end of the collection. This could be caused by issues with the \"\n                            + \"serializer or by calling the map() function more times than there \"\n                            + \"are elements in the collection. Make sure that you set the number \"\n                            + \"of records to be produced by the DataGeneratorSource equal to the \"\n                            + \"number of elements in the collection.\");\n        } catch (Exception e) {\n            throw new IOException(\n                    \"Failed to deserialize an element from the source. \"\n                            + \"If you are using user-defined serialization (Value and Writable \"\n                            + \"types), check the serialization functions.\\nSerializer is \"\n                            + serializer,\n                    e);\n        }\n    }\n\n    private void buildLookup() throws IOException {\n        for (long i = 0; i < numElements; i++) {\n            lookupMap.put(i, tryDeserialize());","sourceCodeStart":129,"sourceCodeEnd":165,"githubUrl":"https://github.com/apache/flink/blob/2f3c205e9266cb30240eb7f4fdab15cad629a70f/flink-connectors/flink-connector-datagen/src/main/java/org/apache/flink/connector/datagen/functions/IndexLookupGeneratorFunction.java#L129-L165","documentation":"Thrown as NoSuchElementException by tryDeserialize in IndexLookupGeneratorFunction during buildLookup() when deserialization hits EOF before all expected numElements are deserialized. The message explains this is caused by serializer issues or by requesting more records than there are elements in the collection.","triggerScenarios":"During open(), buildLookup deserializes numElements items. If the serialized byte array is exhausted early (EOFException), the exception fires. Causes include a TypeSerializer that writes variable-length data inconsistently, a numElements count that does not match the actual serialized content, or a serializer that skips null-valued fields.","commonSituations":"Using a custom serializer that does not round-trip correctly (serialize writes fewer bytes than deserialize reads); TypeInformation mismatch between construction-time serialization and open()-time deserialization after a serialization format change.","solutions":["Verify the TypeSerializer round-trips correctly: serialize N elements, then deserialize exactly N elements.","Ensure the TypeInformation used during construction matches the element types exactly.","Test with a simple known type (e.g., String or Integer) to isolate whether the issue is in the serializer.","Check if the numElements counter (incremented in checkIterable) matches the actual number of serialized elements."],"exampleFix":null,"handlingStrategy":"validation","validationCode":"// Verify round-trip: serialize then deserialize exactly N elements\nTypeSerializer<OUT> serializer = typeInfo.createSerializer(config.getSerializerConfig());\nByteArrayOutputStream baos = new ByteArrayOutputStream();\nDataOutputViewStreamWrapper out = new DataOutputViewStreamWrapper(baos);\nint count = 0;\nfor (OUT element : elements) { serializer.serialize(element, out); count++; }\nDataInputViewStreamWrapper in = new DataInputViewStreamWrapper(\n    new ByteArrayInputStream(baos.toByteArray()));\nfor (int i = 0; i < count; i++) {\n    serializer.deserialize(in); // throws EOFException if mismatch\n}","typeGuard":null,"tryCatchPattern":"try {\n    IndexLookupGeneratorFunction<OUT> fn =\n        new IndexLookupGeneratorFunction<>(typeInfo, elements);\n    fn.open(readerContext);\n} catch (NoSuchElementException e) {\n    if (e.getMessage().startsWith(\"Reached the end of the collection\")) {\n        // serializer mismatch — test round-trip separately\n    }\n    throw e;\n}","preventionTips":["Unit-test TypeSerializer serialize/deserialize round-trip for custom types.","Ensure TypeInformation matches the actual element types exactly.","Verify numElements count matches actual serialized element count."],"tags":["datagen","serialization","deserialization","index-lookup","eof"],"backgroundTag":null,"analyzedSha":"2f3c205e9266cb30240eb7f4fdab15cad629a70f","analyzedAt":"2026-08-14T08:48:24.518Z","schemaVersion":2},"datasetVersion":"2026-08-14T10:17:34.591Z"}