Dependency Inversion Principle in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Dependency Inversion Principle (DIP)

Which statement best describes the Dependency Inversion Principle?

A High-level modules should depend on low-level modules directly to reduce complexity.

B Low-level modules should depend on high-level modules to ensure flexibility.

C Both high-level and low-level modules should depend on abstractions, not on concrete implementations.

D Abstractions should not depend on details, and details should depend on abstractions.

💡 Hint

Think about which direction dependencies should point to reduce tight coupling.

The Dependency Inversion Principle states that both high-level and low-level modules should depend on abstractions (interfaces or abstract classes), not on concrete implementations. This reduces coupling and increases flexibility.

❓ Architecture

intermediate

Applying DIP in a Payment System Design

You are designing a payment system where the high-level module processes payments and the low-level modules handle different payment gateways (e.g., PayPal, Stripe). How should you apply the Dependency Inversion Principle?

A The payment processor directly calls methods of PayPal and Stripe classes.

B The payment processor and payment gateways are tightly coupled for performance.

C Each payment gateway depends on the payment processor to get payment details.

D The payment processor depends on an abstraction (interface) that all payment gateways implement.

💡 Hint

Consider how to decouple the payment processor from specific payment gateway implementations.

By depending on an abstraction that all payment gateways implement, the payment processor can work with any gateway without changing its code, following DIP.

❓ scaling

advanced

Scaling a Modular Notification System Using DIP

You have a notification system with modules for email, SMS, and push notifications. To scale and add new notification types easily, which design aligns best with the Dependency Inversion Principle?

A The notification sender module depends on an abstraction that all notification types implement.

B Each notification type depends on the notification sender module for configuration details.

C The notification sender module depends on concrete classes for email, SMS, and push notifications.

D Notification types are tightly coupled to the sender module to optimize message delivery.

💡 Hint

Think about how to add new notification types without modifying the sender module.

By having the sender depend on an abstraction, new notification types can be added by implementing the abstraction without changing the sender, supporting scalability and DIP.

❓ tradeoff

advanced

Tradeoffs of Applying Dependency Inversion Principle

What is a common tradeoff when applying the Dependency Inversion Principle in system design?

A It reduces flexibility by forcing modules to depend on concrete implementations.

B It can introduce additional complexity due to more abstractions and interfaces.

C It increases coupling between modules, making maintenance harder.

D It eliminates the need for testing modules independently.

💡 Hint

Consider the impact of adding abstractions on system complexity.

While DIP improves flexibility and reduces coupling, it often introduces more interfaces and abstractions, which can increase the system's complexity and learning curve.

❓ component

expert

Identifying DIP Violation in a Code Component

Consider a high-level module OrderProcessor that directly creates and uses a concrete class MySQLDatabase for data storage. Which problem does this violate according to the Dependency Inversion Principle?

A OrderProcessor depends on a concrete low-level module, causing tight coupling and violating DIP.

B MySQLDatabase depends on OrderProcessor, which violates DIP.

C OrderProcessor depends on an abstraction, so it follows DIP correctly.

D There is no violation; direct use of concrete classes is recommended.

💡 Hint

Think about which direction dependencies should point and what modules should depend on.

OrderProcessor depending directly on a concrete class like MySQLDatabase creates tight coupling and violates DIP. Instead, it should depend on an abstraction that MySQLDatabase implements.

Practice

1. What is the main goal of the Dependency Inversion Principle in system design?

easy

A. To make high-level modules independent of low-level modules by using abstractions

B. To tightly couple all modules for better performance

C. To avoid using interfaces or abstractions in the system

D. To make low-level modules dependent on high-level modules

Solution

1. Step 1: Understand the principle's purpose

   The Dependency Inversion Principle aims to reduce direct dependencies between high-level and low-level modules.

2. Step 2: Identify the correct approach

   It achieves this by introducing abstractions or interfaces so that high-level modules do not depend on low-level module details.

3. Final Answer:

   To make high-level modules independent of low-level modules by using abstractions -> Option A

4. Quick Check:

   Dependency Inversion = High-level independent [OK]

Hint: Remember: High-level modules depend on abstractions, not details [OK]

Common Mistakes:

• Thinking low-level modules depend on high-level modules
• Ignoring the use of interfaces or abstractions
• Assuming tight coupling improves design

2. Which of the following code snippets correctly demonstrates the Dependency Inversion Principle?

easy

A. class PaymentProcessor { void process(Payment payment) { payment.pay(); } } interface Payment { void pay(); } class CreditCardPayment implements Payment { void pay() { /* pay with card */ } }

B. class PaymentProcessor { void process(CreditCardPayment payment) { payment.pay(); } } class CreditCardPayment { void pay() { /* pay with card */ } }

C. class PaymentProcessor { void process() { CreditCardPayment payment = new CreditCardPayment(); payment.pay(); } } class CreditCardPayment { void pay() { /* pay with card */ } }

D. interface PaymentProcessor { void process(); } class CreditCardPayment implements PaymentProcessor { void process() { /* pay with card */ } }

Solution

1. Step 1: Identify use of abstraction

   class PaymentProcessor { void process(Payment payment) { payment.pay(); } } interface Payment { void pay(); } class CreditCardPayment implements Payment { void pay() { /* pay with card */ } } uses an interface Payment that both PaymentProcessor and CreditCardPayment depend on, following Dependency Inversion.

2. Step 2: Check other options for tight coupling

   Snippets that tightly couple PaymentProcessor to CreditCardPayment by direct reference or instantiation violate DIP. interface PaymentProcessor { void process(); } class CreditCardPayment implements PaymentProcessor { void process() { /* pay with card */ } } reverses roles incorrectly.

3. Final Answer:

   class PaymentProcessor { void process(Payment payment) { payment.pay(); } } interface Payment { void pay(); } class CreditCardPayment implements Payment { void pay() { /* pay with card */ } } -> Option A

4. Quick Check:

   Use interfaces for dependency inversion [OK]

Hint: Look for interfaces or abstractions connecting modules [OK]

Common Mistakes:

• Directly instantiating concrete classes in high-level modules
• Not using interfaces or abstractions
• Confusing interface roles

3. Given the following code, what will be the output?

interface Logger {
  void log(String message);
}

class ConsoleLogger implements Logger {
  public void log(String message) {
    System.out.println("Console: " + message);
  }
}

class Application {
  private Logger logger;
  public Application(Logger logger) {
    this.logger = logger;
  }
  public void run() {
    logger.log("App started");
  }
}

public class Main {
  public static void main(String[] args) {
    Logger logger = new ConsoleLogger();
    Application app = new Application(logger);
    app.run();
  }
}

medium

A. App started

B. Console: App started

C. Compilation error due to missing method

D. Runtime error due to null pointer

Solution

1. Step 1: Trace object creation and method calls

   Main creates a ConsoleLogger and passes it to Application. Calling app.run() calls logger.log("App started").

2. Step 2: Understand the log method behavior

   ConsoleLogger.log prints "Console: " plus the message, so output is "Console: App started".

3. Final Answer:

   Console: App started -> Option B

4. Quick Check:

   Logger prints with prefix "Console:" [OK]

Hint: Follow object creation and method calls step-by-step [OK]

Common Mistakes:

• Ignoring the prefix in print statement
• Assuming direct print of message without prefix
• Confusing interface with implementation

4. Identify the problem in the following design violating the Dependency Inversion Principle:

class FileStorage {
  public void save(String data) {
    // save data to file
  }
}

class DataManager {
  private FileStorage storage = new FileStorage();
  public void saveData(String data) {
    storage.save(data);
  }
}

medium

A. DataManager should create FileStorage inside saveData method

B. FileStorage should not have a save method

C. There is no problem; design follows Dependency Inversion

D. DataManager depends directly on concrete FileStorage instead of an abstraction

Solution

1. Step 1: Analyze dependencies between classes

   DataManager creates and depends directly on concrete FileStorage, causing tight coupling.

2. Step 2: Check for abstraction usage

   There is no interface or abstraction between DataManager and FileStorage, violating Dependency Inversion Principle.

3. Final Answer:

   DataManager depends directly on concrete FileStorage instead of an abstraction -> Option D

4. Quick Check:

   High-level depends on abstraction, not concrete [OK]

Hint: Look for direct use of concrete classes in high-level modules [OK]

Common Mistakes:

• Thinking method placement is the issue
• Ignoring lack of interfaces
• Assuming creating objects inside methods is correct

5. You are designing a notification system that can send messages via Email, SMS, or Push notifications. To follow the Dependency Inversion Principle, which design is best?

hard

A. Each notifier class depends on NotificationManager to get messages.

B. NotificationManager directly creates and calls methods on EmailNotifier, SMSNotifier, and PushNotifier classes.

C. Create an interface Notifier with method send(message), and implement it in EmailNotifier, SMSNotifier, and PushNotifier. The high-level NotificationManager depends on Notifier interface.

D. NotificationManager contains all sending logic inside itself without separate classes.

Solution

1. Step 1: Identify high-level and low-level modules

   NotificationManager is high-level; EmailNotifier, SMSNotifier, PushNotifier are low-level modules.

2. Step 2: Apply Dependency Inversion Principle

   High-level module should depend on abstraction (Notifier interface), not concrete classes. Create an interface Notifier with method send(message), and implement it in EmailNotifier, SMSNotifier, and PushNotifier. The high-level NotificationManager depends on Notifier interface. follows this by using interface and implementations.

3. Final Answer:

   Use Notifier interface implemented by all notifiers; NotificationManager depends on interface -> Option C

4. Quick Check:

   High-level depends on interface, not concrete classes [OK]

Hint: Use interfaces for all notification types to decouple [OK]

Common Mistakes:

• High-level module directly creating concrete classes
• Putting all logic in one class
• Low-level modules depending on high-level modules