All 56 questions
Which design patterns do you know, and when would you use them?
Asked in 8 of 347 interviews · last asked Oct 14 · Concept · Mid-level
Show answer
I know Strategy, Factory Method, Adapter, and Observer. I use Strategy to swap algorithms, and Factory Method when subclasses should choose which object to create. I use Adapter to make an existing class work with an interface the client expects. I use Observer when several objects need updates after another object changes.
- Factory Method is creational; Adapter is structural; Strategy and Observer are behavioral.
- Choose a pattern for a real problem, not just to use a pattern.
- Patterns add structure, but can also add complexity.
- Explain the problem solved and the trade-offs, not just the pattern name.
What the interviewer checks: The interviewer is checking whether you can connect patterns to practical problems and explain when they help.
Other ways it was asked
- “Which design patterns do you know, and when do you apply them?”
- “Which design patterns are you familiar with?”
- “What do you know about design patterns?”
What is the Singleton design pattern?
Asked in 7 of 347 interviews · last asked Oct 22 · Concept · Junior
Show answer
The Singleton pattern limits a class to one instance and provides a shared way to access it. I would use it when one object must coordinate a shared resource. It controls access, but shared state can hide dependencies and make testing harder.
- The class controls instance creation; callers cannot freely create new instances.
- The instance can be created immediately or only when first needed.
- Concurrent access needs safe initialization; the object's methods may also need synchronization.
- One instance usually means within one application process, not across multiple processes.
What the interviewer checks: The interviewer is checking whether you understand controlled instance creation, suitable uses, and the risks of shared state.
Other ways it was asked
- “What are the advantages and disadvantages of the Singleton design pattern?”
- “What is the singleton pattern?”
- “What is the Singleton design pattern, and when would you use it?”
How does the Decorator design pattern work?
Asked in 5 of 347 interviews · last asked Sep 24 · Concept · Junior
Other ways it was asked
- “In programming, what is a decorator?”
- “What is a decorator, and can you give me an example?”
- “What are decorators?”
How do you decide when to use a design pattern and which one to choose?
Asked in 3 of 347 interviews · last asked Oct 16 · Concept · Mid-level
Other ways it was asked
- “When would you use design patterns in software development?”
- “What are some common software design patterns, and when would you use them?”
- “How do you identify which design pattern to use for an application?”
What is the factory design pattern, and when would you use it?
Asked in 3 of 347 interviews · last asked Sep 29 · Concept · Mid-level
Other ways it was asked
- “What are the Factory and Builder design patterns, and when would you use each?”
- “What is the difference between the Factory and Builder design patterns?”
- “How does the Factory pattern work, and when would you use it?”
What are the main categories of software design patterns?
Asked in 2 of 347 interviews · last asked Feb 14 · Concept · Mid-level
Other ways it was asked
- “What are the main categories of software design patterns, and how do they differ?”
Tell me about a time you used a design pattern to solve a problem. What was the problem, and how did you solve it?
Asked in 2 of 347 interviews · last asked May 11 · Behavioral · Mid-level
Other ways it was asked
- “What is your go-to design pattern? Tell me about a problem you solved recently and which design pattern you used to solve it.”
What are ORM and Active Record, and how are they used to work with databases?
Asked once · last asked Mar 27 · Concept · Mid-level
What design patterns would you use when building APIs with Node.js and Express?
Asked once · last asked Jun 26 · Concept · Mid-level
Other ways it was asked
- “What design patterns have you used when building APIs with Node.js and Express?”
- “Beyond using a consistent format for API responses and errors, what design patterns have you used with Node.js and Express? Can you give an example?”
When using decorators for logging, what should be logged and how can you avoid excessive logging?
Asked once · last asked Feb 23 · Scenario · Mid-level
What approach or framework feature would you use to centralize error handling across 20–30 API endpoints?
Asked once · last asked Mar 5 · Tooling · Mid-level
What design pattern does the RxJS library implement?
Asked once · last asked Mar 24 · Concept · Mid-level
How would you design a notification system that uses user preferences and supports email, SMS, and push notifications without coupling the dispatcher to a specific channel?
Asked once · last asked Mar 31 · Design · Mid-level
What Python design approach would ensure a base-class method runs before each channel-specific email, SMS, or push send implementation?
Asked once · last asked Mar 31 · Concept · Mid-level
Does PDO use the Singleton pattern by default?
Asked once · last asked Apr 9 · Concept · Mid-level
Other ways it was asked
- “What would you do to make PDO work as a Singleton in a PHP application?”
Which backend design concepts or patterns transfer from Java to Python, and what differences matter?
Asked once · last asked Apr 9 · Concept · Mid-level
What is an interceptor in Magento, and what is it used for?
Asked once · last asked Apr 27 · Concept · Mid-level
Can a Magento plugin intercept a protected method?
Asked once · last asked Apr 27 · Concept · Senior
What is the difference between a Factory and a Proxy in Magento?
Asked once · last asked Apr 27 · Concept · Mid-level
In what situations would you create a custom iterator?
Asked once · last asked May 25 · Concept · Mid-level
How can AOP be used to implement cross-cutting logging?
Asked once · last asked Jun 17 · Concept · Senior
What are several design patterns for building multi-step agentic systems?
Asked once · last asked Jun 23 · Design · Senior
What are async/await, LINQ, dependency injection, and common design patterns in C#, and when would you use them?
Asked once · last asked Jun 25 · Concept · Mid-level
What are the Proxy and Decorator design patterns, and how do they differ?
Asked once · last asked Jul 16 · Concept · Mid-level
What patterns or libraries can you use in .NET to separate application components?
Asked once · last asked Jul 16 · Concept · Mid-level
How would you implement a generic object pool for MonoBehaviour components in Unity?
Asked once · last asked Jul 17 · Write code · Mid-level
How would you implement a return method for a generic object pool in Unity?
Asked once · last asked Jul 17 · Write code · Mid-level
How would you implement the Strategy pattern in C# for a database system without switch statements and select the correct strategy at runtime?
Asked once · last asked Aug 4 · Scenario · Senior
Other ways it was asked
- “How would you implement the Strategy pattern in C# without switch statements, and select the correct strategy at runtime?”
How would you handle a missing or unknown strategy key at runtime without disrupting the request flow?
Asked once · last asked Aug 4 · Scenario · Senior
Which backend design patterns or architectural styles do you use, and how do you choose among them?
Asked once · last asked Aug 5 · Concept · Senior
Can you write a Singleton class in Swift?
Asked once · last asked Aug 12 · Write code · Mid-level
Where would you use the Strategy pattern, and what problem does it solve?
Asked once · last asked Aug 25 · Concept · Mid-level
What alternatives to the Singleton pattern would you consider, and when would you use them?
Asked once · last asked Sep 8 · Concept · Mid-level
How can you optimize a Java singleton implementation so it does not create the object eagerly?
Asked once · last asked Sep 11 · Concept · Mid-level
Other ways it was asked
- “How can you make a Java singleton more efficient without synchronizing the entire method?”
Is a lazily initialized Java singleton thread-safe?
Asked once · last asked Sep 11 · Concept · Mid-level
Does the volatile keyword prevent duplicate object creation in a Java singleton?
Asked once · last asked Sep 11 · Concept · Senior
What design patterns can you use to architect front-end code in React?
Asked once · last asked Sep 22 · Concept · Senior
How are design patterns used in Laravel?
Asked once · last asked Sep 23 · Concept · Mid-level
Create a simple Python class that returns only one object throughout the application's lifecycle.
Asked once · last asked Sep 25 · Write code · Mid-level
How does a Python singleton class ensure that only one instance is created during an application's lifecycle?
Asked once · last asked Sep 25 · Concept · Mid-level
How would you design the payment system to support one provider initially and easily add providers like GrabPay, Apple Pay, or Google Pay later?
Asked once · last asked Oct 6 · Design · Mid-level
What is the difference between middleware and interceptor?
Asked once · last asked Oct 7 · Concept · Mid-level
Can you create a skeleton for a singleton class? You don't need to write the complete codebase.
Asked once · last asked Jan 26 · Write code · Mid-level
What does lazy loading mean in software development?
Asked once · last asked Jan 28 · Concept · Junior
Which category of design pattern does Abstract Factory belong to?
Asked once · last asked Feb 14 · Concept · Junior
What kinds of functionality would you wrap with a decorator?
Asked once · last asked Feb 23 · Concept · Junior
Which programming or design patterns do you know? Can you name a few?
Asked once · last asked Apr 16 · Concept · Junior
How can you tell whether an implementation follows the Singleton pattern?
Asked once · last asked May 12 · Concept · Mid-level
How are design patterns classified?
Asked once · last asked May 12 · Concept · Junior
Can you explain the strategy pattern?
Asked once · last asked May 25 · Concept · Junior
What is the delegation pattern, and how is it used in Odoo model inheritance?
Asked once · last asked Jul 23 · Concept · Mid-level
Which design patterns are used in Spring?
Asked once · last asked Jul 27 · Concept · Mid-level
Which design patterns are commonly used in CRM systems?
Asked once · last asked Aug 25 · Concept · Mid-level
What is a design pattern?
Asked once · last asked Sep 3 · Concept · Junior
Does a singleton class in Java need to be final, and why?
Asked once · last asked Sep 11 · Concept · Mid-level
Other ways it was asked
- “Why do you need to make a singleton class final?”
What design patterns are commonly used in Python, and when would you use them?
Asked once · last asked Sep 25 · Concept · Mid-level
More OOP and Design Patterns interview questions
- All OOP and Design Patterns interview questions
- OOP Basics Interview Questions
- Inheritance, Polymorphism, and Interfaces Interview Questions
- SOLID Principles and Object Design Interview Questions
- Dependency Injection and IoC Interview Questions
How this list is made
Every question here was asked by a real interviewer in a real job interview that a candidate ran MeetAssist in. After the call, the interviewer's questions are pulled from the transcript. Practice sessions, tests, exams and anything that was not a job interview are dropped.
Questions are rewritten so they make sense on their own: names of people, employers and clients are removed, and speech-to-text mistakes are fixed. Small talk, logistics and questions about one candidate's own résumé are left out. Rephrasings of the same question are merged, and the count is the number of different interviews it came up in.
The interviews come from candidates who use MeetAssist, so they lean towards software, data and AI roles: the shares describe these interviews, not every OOP and Design Patterns interview. Answers to the most asked questions are written and checked by an editor; a "common mistake" is shown only when reviews of real answers in these interviews show it, with the count.
347 interviews, 240 candidates, Sep 2025 to Oct 2026. Updated every month with new interviews.
Or add the Chrome extension