### Question:
ExampleUserDefinedFunctionHandler extends UserDefinedFunctionHandler { public String concatenate(List<String> list) { StringBuilder sb = new StringBuilder(); for (String value : list) { sb.append(value); } return sb.toString(); } 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 testConcatenateMethod() throws Exception { Schema inputSchema = SchemaBuilder.newBuilder() .addListField("list", Types.MinorType.VARCHAR.getType()) .build(); Schema outputSchema = SchemaBuilder.newBuilder() .addField("string", Types.MinorType.VARCHAR.getType()) .build(); Block inputRecords = allocator.createBlock(inputSchema); inputRecords.setRowCount(1); FieldVector fieldVector = inputRecords.getFieldVector("list"); List<String> value = Lists.newArrayList("a", "b"); BlockUtils.setComplexValue(fieldVector, 0, FieldResolver.DEFAULT, value); UserDefinedFunctionResponse response = runAndAssertSerialization(inputRecords, outputSchema, "concatenate"); Block outputRecords = response.getRecords(); assertEquals(1, outputRecords.getRowCount()); FieldReader fieldReader = outputRecords.getFieldReader("string"); ArrowValueProjector arrowValueProjector = ProjectorUtils.createArrowValueProjector(fieldReader); assertEquals(exampleUserDefinedFunctionHandler.concatenate(Lists.newArrayList("a", "b")), arrowValueProjector.project(0)); }