[FIXED] Sort a List of objects multiple times

Issue

I have a list of objects retrieved through an external REST service that I'd like to sort with user defined values in the Pageable.Sort property.

This is my DTO object:

@Data
@Builder
public class UserDto {
    private String guid;
    private String firstName;
    private String lastName;
}

Given a list of UserDto and a Pageable.Sort. I'm trying to sort the list for each defined attribute and its direction.

public Page<UserDto> listToPage(List<UserDto> list, Pageable pageable) {

        var sorts = pageable.getSort();

        //Retrieves the name of all object fields
        var fields = Arrays.stream(UserDto.class.getDeclaredFields())
                .map(Field::getName)
                .collect(Collectors.toList());

        //For each sort specified in the pageable I need to sort the list of objects
        for (var sort : sorts) {
            //If a given property does not exist. Throws an exception.
            if (!fields.contains(sort.getProperty()))
                throw new InvalidDataException("Property [" + sort.getProperty() + "] does not exists in " + UserDto.class.getName() + " class");

            //Looks for the property and its direction
            switch (sort.getProperty()) {
                case "guid":
                    list = sort.getDirection().isDescending() ?
                            list.stream().sorted(Comparator.comparing(UserDto::getGuid).reversed()).collect(Collectors.toList()) :
                            list.stream().sorted(Comparator.comparing(UserDto::getGuid)).collect(Collectors.toList());
                    break;
                case "firstName":
                    list = sort.getDirection().isDescending() ?
                            list.stream().sorted(Comparator.comparing(UserDto::getFirstName).reversed()).collect(Collectors.toList()) :
                            list.stream().sorted(Comparator.comparing(UserDto::getFirstName)).collect(Collectors.toList());
                    break;
                case "lastName":
                    list = sort.getDirection().isDescending() ?
                            list.stream().sorted(Comparator.comparing(UserDto::getLastName).reversed()).collect(Collectors.toList()) :
                            list.stream().sorted(Comparator.comparing(UserDto::getLastName)).collect(Collectors.toList());
                    break;
                default:
                    break;
            }
        }

        final int start = (int) pageable.getOffset();
        final int end = Math.min((start + pageable.getPageSize()), list.size());
        
        var subList = list.subList(start, end);
        return new PageImpl<>(subList, pageable, list.size());
    }

All works fine, except when I pass multiple attributes to sort the users list.

The first round should sort by firstName:ASC and then, in the second round, it should be sorted by lastName:DESC keeping in mind the first sort order, but this is not doing that.

Is there any way to handle multiple sorts on the same list but in different rounds? I don't want to use thenComparing operator because I want to do this dynamically with user interaction.

A future approach is make this function in a generic way to handle any object and sort its attributes to take advantage of this JPA provided class.

This is my test case:

 @Test
    void testCase1() {
        var userDtoList = List.of(
                UserDto.builder().firstName("A3").lastName("B7").guid("G000a").build(),
                UserDto.builder().firstName("A2").lastName("B4").guid("G000b").build(),
                UserDto.builder().firstName("A3").lastName("B1").guid("G000c").build(),
                UserDto.builder().firstName("A2").lastName("B5").guid("G000d").build(),
                UserDto.builder().firstName("A1").lastName("B6").guid("G000e").build(),
                UserDto.builder().firstName("A2").lastName("B3").guid("G000f").build(),
                UserDto.builder().firstName("A1").lastName("B2").guid("G000g").build()
        );

        Sort firstNameSort = Sort.by("firstName");
        Sort lastNameSort = Sort.by("lastName").descending();

        Sort sort = firstNameSort.and(lastNameSort);

        var pageable = PageRequest.of(0, 5, sort);
        
        var result = userService.listToPage(userDtoList, pageable);
        result.getContent().forEach(System.out::println);

        assertEquals(5, result.getContent().size());

    }

Solution

For someone facing the same issue I solved this creating a generic PageUtils class using Java Reflection.

You can find the code in the following Github repo:

https://github.com/rortegat/generic-pageable

Answered By - rortegat
Answer Checked By - Pedro (JavaFixing Volunteer)