Open/Closed Principle in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Open/Closed Principle

Which statement best describes the Open/Closed Principle in software design?

A Software entities should be open for extension but closed for modification.

B Software entities should be open for modification but closed for extension.

C Software entities should always be rewritten when requirements change.

D Software entities should be neither open for extension nor modification.

💡 Hint

Think about how to add new features without changing existing code.

The Open/Closed Principle means you can add new behavior by extending code, not by changing existing code, which helps avoid bugs.

❓ Architecture

intermediate

Applying Open/Closed Principle in Module Design

You have a payment processing system that currently supports credit cards. You want to add support for new payment methods without changing existing payment code. Which design approach follows the Open/Closed Principle?

A Create a PaymentMethod interface and implement new classes for each payment type, then use polymorphism to process payments.

B Add new if-else conditions in the existing payment processing class for each new payment type.

C Rewrite the entire payment processing class every time a new payment method is added.

D Use global variables to switch payment methods dynamically.

💡 Hint

Think about how to add new payment types without changing existing code.

Using interfaces and polymorphism allows adding new payment types by creating new classes, keeping existing code unchanged.

❓ scaling

advanced

Scaling a Notification System with Open/Closed Principle

You have a notification system that sends alerts via email. You want to add SMS and push notifications without modifying existing email notification code. What is the best scalable design following the Open/Closed Principle?

A Use a single Notification class with many conditional statements for each notification type.

B Add SMS and push notification code directly inside the existing EmailNotification class.

C Duplicate the entire notification system code for each new notification type.

D Create a Notification interface and implement EmailNotification, SMSNotification, and PushNotification classes; use a factory to instantiate them.

💡 Hint

Consider how to add new notification types without changing existing classes.

Using an interface and separate classes for each notification type allows adding new types by extension, not modification, supporting scalability.

❓ tradeoff

advanced

Tradeoffs of Strictly Following Open/Closed Principle

What is a common tradeoff when strictly applying the Open/Closed Principle in system design?

A Reduced code reuse because everything is written from scratch.

B Increased number of classes and interfaces, which can make the system more complex to understand.

C Decreased flexibility since existing code must be modified frequently.

D Lower maintainability due to tightly coupled components.

💡 Hint

Think about how adding extensions affects code structure.

Following Open/Closed Principle often leads to more classes and interfaces, increasing complexity but improving extensibility and safety.

❓ component

expert

Identifying Violation of Open/Closed Principle in Component Design

Given a logging component that writes logs to a file, which change violates the Open/Closed Principle?

Options describe changes to add new logging outputs.

A Creating a new ConsoleLogger class implementing the Logger interface and using it alongside the file logger.

B Adding a new class that implements a Logger interface to support database logging without changing existing file logger code.

C Modifying the existing file logger class to add conditional code for console logging.

D Using a logging manager class that delegates logging to different logger implementations.

💡 Hint

Look for changes that modify existing code instead of extending it.

Modifying existing classes to add new behavior breaks the Open/Closed Principle; new functionality should be added by extension.

Practice

1. What does the Open/Closed Principle (OCP) primarily encourage in software design?

easy

A. Software entities should be open for extension but closed for modification.

B. Software entities should be open for modification but closed for extension.

C. Software entities should be neither open nor closed for changes.

D. Software entities should always be rewritten for new features.

Solution

1. Step 1: Understand the definition of OCP

   The Open/Closed Principle states that software components should allow new functionality to be added without changing existing code.

2. Step 2: Match the principle to the options

   Software entities should be open for extension but closed for modification correctly states that entities should be open for extension (new features) but closed for modification (existing code remains unchanged).

3. Final Answer:

   Software entities should be open for extension but closed for modification. -> Option A

4. Quick Check:

   OCP = Open for extension, closed for modification [OK]

Hint: Remember: extend without changing existing code [OK]

Common Mistakes:

• Confusing 'open for extension' with 'open for modification'
• Thinking OCP means rewriting code for new features
• Ignoring the 'closed for modification' part

2. Which of the following code structures best supports the Open/Closed Principle?

easy

A. Using inheritance or interfaces to add new behavior without changing existing code.

B. Hardcoding all features inside one large class.

C. Copying and pasting code to create new features.

D. Modifying existing classes directly to add new features.

Solution

1. Step 1: Identify structures that allow extension without modification

   Inheritance and interfaces allow new classes to add behavior without changing existing classes.

2. Step 2: Evaluate options against OCP

   Using inheritance or interfaces to add new behavior without changing existing code uses inheritance/interfaces, which aligns with OCP. Other options involve modifying or duplicating code, violating OCP.

3. Final Answer:

   Using inheritance or interfaces to add new behavior without changing existing code. -> Option A

4. Quick Check:

   Inheritance/interfaces = OCP support [OK]

Hint: Choose inheritance or interfaces for extension [OK]

Common Mistakes:

• Thinking direct modification supports OCP
• Believing code duplication is acceptable
• Ignoring design patterns that enable extension

3. Consider this code snippet:

class Shape {
  area() { return 0; }
}

class Rectangle extends Shape {
  constructor(width, height) {
    super();
    this.width = width;
    this.height = height;
  }
  area() { return this.width * this.height; }
}

class Circle extends Shape {
  constructor(radius) {
    super();
    this.radius = radius;
  }
  area() { return Math.PI * this.radius * this.radius; }
}

function totalArea(shapes) {
  return shapes.reduce((sum, shape) => sum + shape.area(), 0);
}

What principle does this design demonstrate?

medium

A. Liskov Substitution Principle by replacing shapes with unrelated classes.

B. Single Responsibility Principle by separating area calculation.

C. Dependency Inversion Principle by depending on concrete classes.

D. Open/Closed Principle by allowing new shapes without modifying totalArea.

Solution

1. Step 1: Analyze how new shapes are added

   New shape classes (Rectangle, Circle) extend Shape and implement area(). totalArea works without changes.

2. Step 2: Match design to OCP

   Since totalArea does not change when new shapes are added, the design follows Open/Closed Principle.

3. Final Answer:

   Open/Closed Principle by allowing new shapes without modifying totalArea. -> Option D

4. Quick Check:

   Adding shapes without changing totalArea = OCP [OK]

Hint: Look for unchanged code when adding new features [OK]

Common Mistakes:

• Confusing OCP with other SOLID principles
• Thinking totalArea must change for new shapes
• Ignoring inheritance usage

4. Given this code violates the Open/Closed Principle:

class Report {
  generate(type) {
    if (type === 'pdf') {
      // generate PDF
    } else if (type === 'html') {
      // generate HTML
    }
  }
}

What is the best fix to comply with OCP?

medium

A. Add more if-else cases inside generate() for new report types.

B. Create separate classes for each report type implementing a common interface.

C. Remove the generate() method and write report code inline.

D. Use global variables to control report type.

Solution

1. Step 1: Identify why current code violates OCP

   Adding new report types requires modifying generate() method, breaking OCP.

2. Step 2: Choose design that allows extension without modification

   Separate classes for each report type implementing a common interface allow adding new types without changing existing code.

3. Final Answer:

   Create separate classes for each report type implementing a common interface. -> Option B

4. Quick Check:

   Separate classes = extend without modify [OK]

Hint: Use separate classes, not if-else chains [OK]

Common Mistakes:

• Adding more if-else blocks inside methods
• Ignoring interface or inheritance use
• Using global variables for behavior control

5. You are designing a payment system that must support new payment methods without changing existing code. Which approach best follows the Open/Closed Principle?

hard

A. Rewrite the payment processor each time a new method is added.

B. Add new payment method code inside a large switch statement in the payment processor.

C. Implement a Payment interface and create new classes for each payment method.

D. Use global flags to select payment methods inside one class.

Solution

1. Step 1: Understand the requirement for extensibility without modification

   The system must allow adding new payment methods without changing existing code.

2. Step 2: Evaluate design options for OCP compliance

   Implementing a Payment interface and creating new classes for each method allows extension by adding classes, not modifying existing ones.

3. Final Answer:

   Implement a Payment interface and create new classes for each payment method. -> Option C

4. Quick Check:

   Interface + new classes = OCP compliance [OK]

Hint: Use interfaces and new classes for new features [OK]

Common Mistakes:

• Using switch statements that require modification
• Rewriting existing code repeatedly
• Using global flags for behavior control