Object Oriented Design: Dependency Inversion Principle (DIP)

The Dependency Inversion Principle (DIP) is one of the SOLID principles of object-oriented design and it emphasizes the importance of decoupling in software architecture.

Dependency Inversion Principle states that:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
2. Abstractions should not depend on details. Details should depend on abstractions.

Let’s illustrate with an example:

Without Dependency Inversion:

Imagine you have a light bulb class and a switch to operate it.

class LightBulb {
    void turnOn() {
        System.out.println("LightBulb: Bulb turned on...");
    }

    void turnOff() {
        System.out.println("LightBulb: Bulb turned off...");
    }
}

class Switch {
    private LightBulb lightBulb;

    public Switch(LightBulb lightBulb) {
        this.lightBulb = lightBulb;
    }

    void operate() {
        // some logic to turn on or turn off
    }
}

In the above code, Switch class directly depends on the LightBulb class. If we were to introduce another device, like a fan, the Switch class would need changes, making it less flexible and violating the open/closed principle.

With Dependency Inversion:

We introduce an interface (Switchable) that both the high-level module (Switch) and the low-level modules (LightBulb, Fan) depend upon.

interface Switchable {
    void turnOn();
    void turnOff();
}

class LightBulb implements Switchable {
    @Override
    public void turnOn() {
        System.out.println("LightBulb: Bulb turned on...");
    }

    @Override
    public void turnOff() {
        System.out.println("LightBulb: Bulb turned off...");
    }
}

class Fan implements Switchable {
    @Override
    public void turnOn() {
        System.out.println("Fan: Fan turned on...");
    }

    @Override
    public void turnOff() {
        System.out.println("Fan: Fan turned off...");
    }
}

class Switch {
    private Switchable device;

    public Switch(Switchable device) {
        this.device = device;
    }

    void operate() {
        // some logic to turn on or turn off
    }
}

With this revised design:

• Switch doesn't directly depend on LightBulb or Fan. It depends on the Switchable abstraction, making it decoupled from the specific devices.
• Introducing new devices or changing existing devices has minimal impact on the Switch class.
• The high-level Switch module and low-level modules (LightBulb, Fan) both depend on the Switchable abstraction, adhering to the Dependency Inversion Principle.

This approach makes the system more flexible, extensible, and easier to maintain.

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