The Data Mapper is a design pattern that proposes an intermediary component to map data between the database and the application's business objects, without the latter having any knowledge of the database.

This allows for a clear separation between business logic (domain objects) and data access, facilitating the independent maintenance and evolution of both.

# When to use it?

This pattern is ideal in applications where the database structure and the application's domain logic differ significantly, or when it is desired to keep business logic completely decoupled from data access code.

It is especially useful in complex systems, where this separation and abstraction facilitate the management of changes in both the data model and business rules without one directly affecting the other.

# Detailed Example in Java

A Data Mapper for Person would be responsible for transferring data between Person instances and the people table without the Person class knowing anything about storage.

public class PersonMapper {
    private Database db; // Assume this is an abstraction for database management

    public PersonMapper(Database db) {
        this.db = db;
    }

    public Person findById(int id) {
        // Performs a database query and constructs a Person instance with the results
    }

    public void save(Person person) {
        // Checks if the person already exists in the database and performs an insertion or update
    }

    // Other methods for updating, deleting, etc.
}