The purpose of the Converter pattern is to provide a generic, common way of bidirectional conversion between corresponding types, allowing a clean implementation in which the types do not need to be aware of each other. Moreover, the Converter pattern introduces bidirectional collection mapping, reducing a boilerplate code to minimum.
Real world example
In real world applications it is often the case that database layer consists of entities that need to be mapped into DTOs for use on the business logic layer. Similar mapping is done for potentially huge amount of classes and we need a generic way to achieve this.
In plain words
Converter pattern makes it easy to map instances of one class into instances of another class.
Programmatic Example
We need a generic solution for the mapping problem. To achieve this, let’s introduce a generic converter.
1public class Converter<T, U> {
2
3 private final Function<T, U> fromDto;
4 private final Function<U, T> fromEntity;
5
6 public Converter(final Function<T, U> fromDto, final Function<U, T> fromEntity) {
7 this.fromDto = fromDto;
8 this.fromEntity = fromEntity;
9 }
10
11 public final U convertFromDto(final T dto) {
12 return fromDto.apply(dto);
13 }
14
15 public final T convertFromEntity(final U entity) {
16 return fromEntity.apply(entity);
17 }
18
19 public final List<U> createFromDtos(final Collection<T> dtos) {
20 return dtos.stream().map(this::convertFromDto).collect(Collectors.toList());
21 }
22
23 public final List<T> createFromEntities(final Collection<U> entities) {
24 return entities.stream().map(this::convertFromEntity).collect(Collectors.toList());
25 }
26}
The specialized converters inherit from this base class as follows.
1public class UserConverter extends Converter<UserDto, User> {
2
3 public UserConverter() {
4 super(UserConverter::convertToEntity, UserConverter::convertToDto);
5 }
6
7 private static UserDto convertToDto(User user) {
8 return new UserDto(user.getFirstName(), user.getLastName(), user.isActive(), user.getUserId());
9 }
10
11 private static User convertToEntity(UserDto dto) {
12 return new User(dto.getFirstName(), dto.getLastName(), dto.isActive(), dto.getEmail());
13 }
14
15}
Now mapping between User
and UserDto
becomes trivial.
1var userConverter = new UserConverter();
2var dtoUser = new UserDto("John", "Doe", true, "whatever[at]wherever.com");
3var user = userConverter.convertFromDto(dtoUser);
Use the Converter Pattern in the following situations: