### Question:
ExampleUserDefinedFunctionHandler extends UserDefinedFunctionHandler { public String to_json(Map<String, Object> struct) { try { return objectMapper.writeValueAsString(struct); } catch (JsonProcessingException e) { throw new RuntimeException(e); } } ExampleUserDefinedFunctionHandler(); Integer multiply(Integer factor1, Integer factor2); String concatenate(List<String> list); String to_json(Map<String, Object> struct); Long get_default_value_if_null(Long value);  }### Answer:
@Test public void testToJsonMethod() throws Exception { Schema inputSchema = SchemaBuilder.newBuilder() .addStructField("struct") .addChildField("struct", "int", Types.MinorType.INT.getType()) .addChildField("struct", "double", Types.MinorType.FLOAT8.getType()) .addChildField("struct", "string", Types.MinorType.VARCHAR.getType()) .build(); Schema outputSchema = SchemaBuilder.newBuilder() .addField("json", Types.MinorType.VARCHAR.getType()) .build(); Block inputRecords = allocator.createBlock(inputSchema); inputRecords.setRowCount(1); FieldVector fieldVector = inputRecords.getFieldVector("struct"); Map<String, Object> struct = new HashMap<>(); struct.put("int", 10); struct.put("double", 2.3); struct.put("string", "test_string"); BlockUtils.setComplexValue(fieldVector, 0, FieldResolver.DEFAULT, struct); UserDefinedFunctionResponse response = runAndAssertSerialization(inputRecords, outputSchema, "to_json"); Block outputRecords = response.getRecords(); assertEquals(1, outputRecords.getRowCount()); FieldReader fieldReader = outputRecords.getFieldReader("json"); ArrowValueProjector arrowValueProjector = ProjectorUtils.createArrowValueProjector(fieldReader); assertEquals(exampleUserDefinedFunctionHandler.to_json(struct), arrowValueProjector.project(0)); }