Converter Pattern

In this article, we will learn how to use and implement a Converter Pattern in Java with an example.

Intent

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 a minimum.

Explanation

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.

Class diagram

Let's take a look at the class diagram of Converter Pattern implementation in Java:

Converter Pattern Implementation in Java Example

We need a generic solution for the mapping problem. To achieve this, let's introduce a generic converter.
public class Converter<T, U> {

  private final Function<T, U> fromDto;
  private final Function<U, T> fromEntity;

  public Converter(final Function<T, U> fromDto, final Function<U, T> fromEntity) {
    this.fromDto = fromDto;
    this.fromEntity = fromEntity;
  }

  public final U convertFromDto(final T dto) {
    return fromDto.apply(dto);
  }

  public final T convertFromEntity(final U entity) {
    return fromEntity.apply(entity);
  }

  public final List<U> createFromDtos(final Collection<T> dtos) {
    return dtos.stream().map(this::convertFromDto).collect(Collectors.toList());
  }

  public final List<T> createFromEntities(final Collection<U> entities) {
    return entities.stream().map(this::convertFromEntity).collect(Collectors.toList());
  }
}
The specialized converters inherit from this base class as follows.
public class UserConverter extends Converter<UserDto, User> {

  public UserConverter() {
    super(UserConverter::convertToEntity, UserConverter::convertToDto);
  }

  private static UserDto convertToDto(User user) {
    return new UserDto(user.getFirstName(), user.getLastName(), user.isActive(), user.getUserId());
  }

  private static User convertToEntity(UserDto dto) {
    return new User(dto.getFirstName(), dto.getLastName(), dto.isActive(), dto.getEmail());
  }

}
Now mapping between User and UserDto becomes trivial.
var userConverter = new UserConverter();
var dtoUser = new UserDto("admin", "admin", true, "whatever[at]wherever.com");
var user = userConverter.convertFromDto(dtoUser);

Applicability

Use the Converter Pattern in the following situations:
  • When you have types that logically correspond which other and you need to convert entities between them
  • When you want to provide different ways of types conversions depending on a context
  • Whenever you introduce a DTO (Data transfer object), you will probably need to convert it into the domain equivalence

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