Dependency Inversion Principle (DIP)

Consider a scenario where a class directly depends on a low-level implementation:

class MySQLDatabase {
    void save(String data) {
        System.out.println("Saving data to MySQL database");
    }
}

class Application {
    private MySQLDatabase database = new MySQLDatabase();

    void storeData(String data) {
        database.save(data);
    }
}
            

This design violates DIP because the Application class depends directly on the MySQLDatabase implementation. To adhere to DIP, introduce an abstraction:

interface Database {
    void save(String data);
}

class MySQLDatabase implements Database {
    public void save(String data) {
        System.out.println("Saving data to MySQL database");
    }
}

class Application {
    private Database database;

    Application(Database database) {
        this.database = database;
    }

    void storeData(String data) {
        database.save(data);
    }
}
            

Now, the Application class depends on the Database interface, allowing flexibility to switch to a different implementation (e.g., PostgreSQL, MongoDB) without modifying the application logic.