Dependency Inversion

High-level modules should not depend on low-level modules. Both should depend on abstractions.

Explanation: Instead of directly depending on concrete implementations, depend on abstractions (interfaces).

Java Example

Bad Design (Violating DIP)

class MySQLDatabase {
    void connect() {
        System.out.println("Connecting to MySQL...");
    }
}

class Application {
    MySQLDatabase database = new MySQLDatabase();

    void start() {
        database.connect();
    }
}

Here, Application is tightly coupled to MySQLDatabase, making it hard to switch to another database.

Good Design (Following DIP)

interface Database {
    void connect();
}

class MySQLDatabase implements Database {
    public void connect() {
        System.out.println("Connecting to MySQL...");
    }
}

class PostgreSQLDatabase implements Database {
    public void connect() {
        System.out.println("Connecting to PostgreSQL...");
    }
}

class Application {
    private Database database;

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

    void start() {
        database.connect();
    }
}

Now, Application depends on the Database interface, making it flexible to use any database.

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Python Example

Bad Design (Violating DIP)

class MySQLDatabase {
    public void connect() {
        System.out.println("Connecting to MySQL...");
    }
}

class Application {
    private MySQLDatabase database;
    
    public Application() {
        this.database = new MySQLDatabase();
    }
    
    public void start() {
        this.database.connect();
    }
}

👎 Problem: The Application class is tightly coupled to MySQLDatabase. Switching to another database requires modifying the application.

Good Design (Following DIP)

// Database interface
interface Database {
    void connect();
}

class MySQLDatabase implements Database {
    @Override
    public void connect() {
        System.out.println("Connecting to MySQL...");
    }
}

class PostgreSQLDatabase implements Database {
    @Override
    public void connect() {
        System.out.println("Connecting to PostgreSQL...");
    }
}

class Application {
    private Database database;
    
    public Application(Database database) {
        this.database = database;
    }
    
    public void start() {
        this.database.connect();
    }
}

👍 Fix: Now, Application depends on the Database interface, making it easy to switch databases.