How can I change parameterized type in collection?

December 2, 2022

I would like to use collection in one method in my UserServiceImpl to return the list of all users which will be parameterized by UserDTO.
I have the following method:

@Override
public List<UserEntity> getUsers() {
    var usersList = userRepository.findAll();
    return usersList;
}

But I want to change it to public List<UserDTO> getUsers()... I have map method from entity to dto and vice versa:

public UserDTO mapToUserDTO(UserEntity userEntity) {
    var userDto = new UserDTO();
    var rolesEntity = userEntity.getRoles().stream()
            .map(RoleEntity::getId)
            .map(String::valueOf)
            .collect(Collectors.toList());
    userDto.setId(userEntity.getId());
    userDto.setUsername(userEntity.getUsername());
    userDto.setName(userEntity.getName());
    userDto.setSurname(userEntity.getSurname());
    userDto.setEmail(userEntity.getEmail());
    userDto.setAge(userEntity.getAge());
    userDto.setRoles(rolesEntity);
    return userDto;
}

But in this case it cannot be applied.
Could you help me out please – how can I change parametrized type from UserEntity to UserDTO in my method?

>Solution :

Create a stream over the list of user entities, transform each entity into a DTO via map() operation and the method you’ve shared and collect the result into a list.

public List<UserDTO> getUsers() {
    
    return userRepository.findAll().stream()
        .map(this::mapToUserDTO)       // or ClassName::mapToUserDTO
        .collect(Collectors.toList()); // or .toList() for Java 16+
}