@Document
public class Employee
{
 @Id
 Long empCode;
 String address;
// getters setters
}

public interface EmployeeRepository extends MongoRepository<Employee, Long>
{

List<Employee> findEmployeesByEmpCode(int empCode);

@Query(value="{ 'empCode' : ?0 }", fields="{ 'address' : 1}")
List<String> findAddressByEmpCode(int empCode);

java.lang.IllegalArgumentException: [Assertion failed] - this argument is required; it must not be null
    at org.springframework.util.Assert.notNull(Assert.java:115)
    at org.springframework.util.Assert.notNull(Assert.java:126)
    at org.springframework.data.convert.EntityInstantiators.getInstantiatorFor(EntityInstantiators.java:86)
    at org.springframework.data.mongodb.repository.query.DtoInstantiatingConverter.<init>(DtoInstantiatingConverter.java:61)
    at org.springframework.data.mongodb.repository.query.MongoQueryExecution$ResultProcessingConverter.convert(MongoQueryExecution.java:376)
    at org.springframework.data.mongodb.repository.query.MongoQueryExecution$ResultProcessingExecution.execute(MongoQueryExecution.java:345)
    at org.springframework.data.mongodb.repository.query.AbstractMongoQuery.execute(AbstractMongoQuery.java:91)