Problem Statement:

You are tasked with designing a system that adheres to the Dependency Inversion Principle (DIP) from the SOLID principles. The Dependency Inversion Principle states that high-level modules should not depend on low-level modules; both should depend on abstractions. Abstractions should not depend on details; details should depend on abstractions. The goal is to create a system where the high-level modules are not directly dependent on the low-level modules, promoting flexibility and ease of maintenance.

Solution:

1. Define Abstractions (Interfaces):

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

   public interface Device {
       void operate();
   }

2. Create Low-Level Modules Implementing Abstractions:

   public class LightBulb implements Switchable {
       @Override
       public void turnOn() {
           System.out.println("Light bulb is on.");
       }
   
       @Override
       public void turnOff() {
           System.out.println("Light bulb is off.");
       }
   }

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

   public class DeviceController implements Device {
       private Switchable device;
   
       public DeviceController(Switchable device) {
           this.device = device;
       }
   
       @Override
       public void operate() {
           device.turnOn();
           device.turnOff();
       }
   }

3. Example Usage:

   public class Main {
       public static void main(String[] args) {
           Switchable lightBulb = new LightBulb();
           Switchable fan = new Fan();
   
           DeviceController lightBulbController = new DeviceController(lightBulb);
           DeviceController fanController = new DeviceController(fan);
   
           lightBulbController.operate();
           fanController.operate();
       }
   }