Open/Closed Principle in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Open/Closed Principle in software design?

It means software entities like classes or modules should be open for extension but closed for modification. You can add new features without changing existing code.

beginner

Why is the Open/Closed Principle important?

It helps keep code stable and reduces bugs by avoiding changes to tested code. It also makes adding new features easier and safer.

intermediate

How can you apply the Open/Closed Principle in a system?

Use interfaces or abstract classes to define behavior, then add new classes to extend functionality without changing existing ones.

beginner

Give a real-life example of the Open/Closed Principle.

Think of a smartphone app that supports new payment methods by adding new modules, without changing the core app code.

intermediate

What can happen if you ignore the Open/Closed Principle?

You risk breaking existing features when adding new ones, making the system fragile and harder to maintain.

What does 'closed for modification' mean in the Open/Closed Principle?

A You should not change existing tested code.

B You should never add new features.

C You must rewrite code for every change.

D You should delete old code frequently.

Closed for modification means existing code should not be changed to avoid introducing bugs.

How do you make a system 'open for extension'?

A By avoiding new features.

B By rewriting all code from scratch.

C By deleting old features.

D By adding new code without changing existing code.

Open for extension means you can add new features by adding new code, not changing old code.

Which design tool helps implement the Open/Closed Principle?

A Interfaces or abstract classes

B Global variables

C Hard-coded values

D Copy-pasting code

Interfaces and abstract classes allow adding new behaviors without modifying existing code.

Ignoring the Open/Closed Principle can lead to:

A More stable software

B Fragile code that breaks easily

C Faster development always

D Less bugs automatically

Changing existing code often introduces bugs, making the system fragile.

Which of these is a good practice for Open/Closed Principle?

A Avoid using interfaces

B Modify existing classes for new features

C Add new classes to extend behavior

D Write all code in one big class

Adding new classes to extend behavior keeps existing code unchanged.

Explain the Open/Closed Principle and why it matters in software design.

Think about how you can add features without breaking existing code.

Describe a simple example where applying the Open/Closed Principle improves a system.

Imagine adding a new payment method to an app without changing old code.