A Mapper is a component responsible for transferring data between two objects or systems that should not know about each other directly. Its main function is to decouple the different data representations in an application.

It is often used to transform domain objects into DTOs (Data Transfer Objects) for transmission via web services, or to map data from a database to business objects.

# When to use it?

It is extremely useful when the data structures in different layers of your application differ, or when you need to integrate external systems with their own data formats.

# Detailed Example in Java

An example of a Mapper to convert a User Entity into a DTO for the API:

public class UserMapper {
    public UserDTO toDTO(User user) {
        return new UserDTO(
            user.getId(),
            user.getFirstName() + " " + user.getLastName(),
            user.getEmail()
        );
    }

    public User toEntity(UserDTO dto) {
        // Logic to rebuild the entity from the DTO
    }
}