4,100 Backend Developer Interview Questions from Real Interviews

What should I focus on for a backend interview?

Which topics stand out for backend roles?

Start with system design and databases. System design appeared in 49% of these interviews, compared with 20% across all interviews. Relational databases, concurrency, REST API design, algorithms, and performance work were also more common here.

Mid-level questions made up the largest share, but junior and senior questions were also present. Check specific choices: when would you use a database index? When would you choose microservices over a monolith? Can you explain both the choice and its costs?

Which common questions should I check first?

Processes versus threads was the most asked question, appearing in 21 of 587 interviews. Can you explain how they differ and when you would choose each?

Dependency injection, OOP principles, and microservices were also among the most asked questions. Use these checks:

Can you explain how a dependency injection container works?
Can you explain the main OOP principles?
Can you explain method overloading versus method overriding?
If you use Python, can you explain how a list differs from a tuple?

What did reviewed design answers miss?

Some reviewed dependency injection answers did not explain how it reduces coupling or supports tests with mocks or stubs. Can you explain how it makes code less tied to its dependencies and lets you replace them in tests?

For microservices, reviewed answers missed the balance between independent deployment and team freedom, and the added work of running services and handling communication between them. Can you cover both sides?

For interfaces and abstract classes, the missing detail was when to choose each in a real design. Can you explain your choice, not just list differences?

Can I write the code, not just describe it?

Coding tasks appeared in 144 of 587 interviews. Some reviewed Two Sum answers had no working function. Can you write one that finds a pair adding up to the target, returns its indices, and explain its time complexity?

Also check whether you can:

Write SQL to find the second-highest salary.
Reverse a string in code.
Find the largest value in an array.
Write code for the Two Sum problem.

Which technical details should I check?

These details were missing from reviewed answers. Can you explain each one?

A FULL OUTER JOIN keeps matched rows and unmatched rows from both tables, using NULL for missing values.
JWT authentication involves issuing a token after login, sending it with requests, and validating it on the server.
Java's HashMap uses a key's hash to select a bucket.

What Backend Developer interviews cover

System design & architecture49% of interviews

Relational databases37% of interviews

Data structures & algorithms29% of interviews

REST API design29% of interviews

Performance optimization & profiling29% of interviews

Concurrency & multithreading28% of interviews

The 20 most asked Backend Developer questions

Counted by interview, not by mention: a question asked twice in one conversation counts once.

How do processes and threads differ, and when would you choose one over the other?
Asked in 21 of 587 interviews · last asked Oct 23 · Concept · Junior
Show answer
Processes have separate memory spaces, while threads share memory within one process. I choose threads for concurrent I/O or shared data, usually with less overhead. I choose processes for stronger isolation or CPU parallelism when the language runtime limits threads. Shared data needs careful synchronization to avoid race conditions.
- Processes usually cost more to start and need explicit communication to exchange data.
- Threads share memory, so concurrent changes may need locks or other synchronization.
- A process crash can stop all its threads; separate processes provide better fault isolation.
- CPU parallelism with threads depends on the language runtime and available CPU cores.
What the interviewer checks: The interviewer checks whether you can balance memory sharing, overhead, isolation, and parallelism for a workload.
Other ways it was asked
- “What is the difference between a process and a thread?”
- “When would you choose multiple processes instead of multiple threads?”
- “What is the difference between a thread and a process?”
What is dependency injection, and how does a dependency injection container work?
Asked in 18 of 587 interviews · last asked Oct 21 · Concept · Junior
Show answer
Dependency injection means a class receives the objects it needs from outside instead of creating them itself. This reduces coupling and makes testing with mocks or stubs easier. A dependency injection container uses registrations or configuration to find, create, and supply those objects. It also manages their lifetimes, such as creating one per request or sharing one instance.
- Dependencies are often passed through a constructor, making the class's needs clear.
- A container resolves dependencies, including dependencies needed by other dependencies.
- Tests can supply mocks or stubs instead of real databases or external services.
- Dependency injection does not require a container; you can pass objects yourself.
What the interviewer checks: The interviewer checks whether you understand how dependencies are supplied, how containers manage them, and why this improves testing.
Common mistake (in 4 of 10 reviewed answers): Many answers forget that receiving dependencies from outside reduces coupling and lets tests replace them with mocks or stubs.
Other ways it was asked
- “What is dependency injection?”
- “What is dependency injection, and why is it used in application design?”
- “What is dependency injection, and how does it work?”
What is object-oriented programming, and what are its four main principles?
Asked in 18 of 587 interviews · last asked Oct 22 · Concept · Junior
Show answer
Object-oriented programming (OOP) organizes code around objects that combine data and behavior. Its four main principles are encapsulation, abstraction, inheritance, and polymorphism.
- Encapsulation groups data with methods and controls access to an object's internal state.
- Abstraction exposes what an object can do while hiding how it works.
- Inheritance lets a class reuse or extend another class's behavior.
- Polymorphism lets different objects respond differently through the same interface.
What the interviewer checks: The interviewer checks whether you understand objects and can explain each principle, not just name it.
Other ways it was asked
- “What is object-oriented programming?”
- “What are the core principles of object-oriented programming?”
- “What is OOP?”
What are microservices, and when would you choose them over a monolith?
Asked in 18 of 587 interviews · last asked Oct 16 · Design · Senior
Show answer
Microservices split an application into small services that can be deployed independently. I would choose them when separate teams need to release or scale parts independently, and can manage distributed systems. Otherwise, I would choose a monolith, often with clear internal modules, because it is simpler to build, test, and run.
- Split services around business responsibilities, not just technical layers.
- Independent releases give teams more control; separate scaling can target busy services.
- Network calls add delays and failures; data consistency and debugging become harder.
- Consider team size, monitoring, deployment tools, and operating costs before choosing microservices.
What the interviewer checks: The interviewer checks whether you can match the architecture to business needs and team capacity while explaining the trade-offs.
Common mistake (in 3 of 8 reviewed answers): Many answers forget that independent deployment and team control come with more operational work and cross-service communication costs.
Other ways it was asked
- “What are microservices?”
- “What are microservices, and when would you use a microservices architecture?”
- “What are microservices, and when would you use them?”
What is the difference between a list and a tuple in Python?
Asked in 16 of 587 interviews · last asked Oct 17 · Concept · Junior
Show answer
A list is mutable: you can add, remove, or replace items. A tuple is immutable: you cannot change which objects it holds, which prevents accidental changes to its structure. Use a list for a changing collection and a tuple for a fixed record. Lists are unhashable; tuples are hashable only if all their items are hashable.
- Both keep item order and support indexing and slicing.
- A mutable object inside a tuple, such as a list, can still change.
- A hashable tuple can be a dictionary key or a set member; a list cannot.
What the interviewer checks: The interviewer checks whether you understand mutability, hashability, and when to choose each type.
Common mistake (in 3 of 6 reviewed answers): Many answers forget that tuple immutability prevents structural changes, but does not freeze mutable objects inside it.
Other ways it was asked
- “What is the difference between a list and a tuple?”
- “In Python 3, what is the difference between a list and a tuple in terms of mutability and hashability?”
- “In Python, what is the difference between a list and a tuple, and when would you choose one over the other?”
Write a function that solves the Two Sum problem. What is the time complexity of your solution?
Asked in 16 of 587 interviews · last asked Oct 23 · Write code · Junior
Show answer
I use a hash map to find two numbers that add up to the target and return their indices. For each number, I check whether its needed partner is already in the map. This takes O(n) expected time and O(n) extra space. I return None if no pair exists.
- Store each visited number and its index in a hash map.
- Check for the partner before storing the current number, so the same index is not used twice.
- Duplicate values work if they appear at different indices.
- Hash map lookups and inserts take O(1) average time, not guaranteed worst-case time.
Python
```
def two_sum(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        partner = target - num
        if partner in seen:
            return [seen[partner], i]
        seen[num] = i
    return None
```
What the interviewer checks: The interviewer checks whether you can write working code, use a hash map, and explain time and space costs.
Common mistake (in 4 of 5 reviewed answers): Many answers forget that explaining the approach is not enough; write a working function that returns the pair's indices.
Other ways it was asked
- “Write a function that solves the Two Sum problem.”
- “What is the time complexity of your Two Sum solution?”
- “What is the time complexity of a solution to the Two Sum problem?”
How do you use AI tools in your day-to-day software development?
Asked in 15 of 587 interviews · last asked Oct 23 · Tooling
Show answer
I use AI to draft routine code, suggest tests, explain unfamiliar code, and explore possible causes of bugs. I give it a small, clear task, then review its output and run tests before using it. I use approved tools and keep secrets and restricted data out of prompts. I stay responsible for the final code.
- Use AI for clear tasks, not as a replacement for your own judgment.
- Check generated code for correctness, security, and fit with the existing system.
- Verify suggested APIs and library behavior against current documentation.
- Follow company rules on sharing code and data.
What the interviewer checks: The interviewer wants to know whether you use AI practically while protecting data and keeping responsibility for code quality.
Other ways it was asked
- “How are you using AI tools in your software development work?”
- “How do you use AI in your daily software development work, and how does it fit into your overall process?”
- “Which AI tools do you use in software development, and how do you use them?”
What are database indexes, and when would you use them?
Asked in 14 of 587 interviews · last asked Oct 14 · Concept · Junior
Show answer
Database indexes are data structures that help a database find rows without scanning the whole table. I use them on columns often used in filters, joins, or sorting. They can speed up reads, but they use storage and add work to inserts, updates, and deletes.
- Choose indexes based on frequent queries, not every column.
- An index may not help when a query reads most of a table.
- Column order matters in indexes that cover multiple columns.
- Check query execution plans to see whether indexes help.
What the interviewer checks: The interviewer is checking whether you understand how indexes speed up reads and their storage and write costs.
Other ways it was asked
- “What are database indexes, and what are they used for?”
- “What is indexing?”
- “What are database indexes, and how do they affect database performance?”
What are method overloading and method overriding?
Asked in 12 of 587 interviews · last asked Oct 22 · Concept · Junior
Show answer
In Java, method overloading means using the same method name with different parameter lists. Method overriding means a subclass provides a new implementation of an inherited instance method. Overload selection happens at compile time; the overridden implementation is chosen at runtime based on the actual object.
- Overloads differ by parameter count, types, or order—not by return type alone.
- An override keeps the same method signature and must have a compatible return type.
- An override cannot reduce access or declare broader checked exceptions.
- Static methods are hidden, not overridden; final and private methods cannot be overridden.
What the interviewer checks: The interviewer is checking whether you understand method signatures, inheritance, and compile-time selection versus runtime dispatch.
Other ways it was asked
- “What is the difference between method overloading and method overriding?”
- “What is method overriding in Java?”
- “What is method overloading in Java?”
What is the difference between an interface and an abstract class?
Asked in 12 of 587 interviews · last asked Oct 21 · Concept · Junior
Show answer
In Java, an interface defines a contract, while an abstract class can also hold instance state, constructors, and shared code. A class can implement several interfaces but extend only one class. Choose an interface for a shared capability across unrelated classes. Choose an abstract class when related classes need shared state or setup.
- Neither can be instantiated directly.
- Modern Java interfaces can include default and static methods, but have no instance fields or constructors.
- Abstract classes can mix abstract methods with implemented methods.
- Abstraction is a design idea; interfaces and abstract classes are ways to express it.
What the interviewer checks: The interviewer checks whether you understand Java inheritance rules and can choose the right tool for a design.
Common mistake (in 3 of 5 reviewed answers): Many answers forget that the choice should depend on shared capabilities versus shared state and setup.
Other ways it was asked
- “What is the difference between an abstract class and an interface?”
- “What is the difference between an abstract class and an interface, and why would you use an abstract class instead of an interface?”
- “What is the difference between abstraction and an interface?”
What is polymorphism, and how does it work in Java?
Asked in 12 of 587 interviews · last asked Oct 17 · Concept · Junior
Show answer
Polymorphism lets the same method call behave differently for different object types. In Java, a superclass or interface reference can hold a subtype object, and Java chooses its overridden instance method at runtime. Method overloading is also called compile-time polymorphism: Java selects a method signature using the declared argument types.
- Overriding gives a subclass its own implementation of an inherited method.
- Runtime dispatch uses the actual object type, not the reference type.
- Overloading means methods share a name but have different parameter lists.
- Static methods are hidden, not overridden.
Code
```
public class Main {
    static class Animal { void speak() { System.out.println("Sound"); } }
    static class Dog extends Animal {
        @Override void speak() { System.out.println("Woof"); }
    }
    public static void main(String[] args) {
        Animal animal = new Dog();
        animal.speak(); // Prints Woof
    }
}
```
What the interviewer checks: The interviewer checks whether you understand runtime method selection and can distinguish overriding from overloading.
Other ways it was asked
- “What is polymorphism?”
- “What is polymorphism in Java?”
- “Can you tell me what polymorphism is?”
How would you design payment idempotency, including key storage, validation, and responses, to prevent duplicate charges?
Asked in 12 of 587 interviews · last asked Oct 22 · Design · Senior
Show answer
I would store each idempotency key in a durable database, scoped to the caller and operation, with a unique constraint. I would save the request fingerprint, payment state, and response. Reusing a key with different payment details fails; completed retries receive the saved response. Pending retries return a pending status, not another charge.
- Claim keys atomically; commit local balance changes and completion together in one database transaction.
- Use provider idempotency where supported; otherwise reconcile uncertain results before attempting another charge.
- Keep a permanent unique payment ID if messages can arrive after idempotency keys expire.
- Ignore duplicate or older events; never move a settled payment back to a pending state.
What the interviewer checks: The interviewer checks whether your design prevents duplicate payments during concurrent requests, restarts, timeouts, and late events.
Other ways it was asked
- “If a service restarts and an external service resends the same message for a 50,000-peso payment, how would you prevent the recipient from receiving 100,000 pesos?”
- “If an external service resends the same 50,000-peso payment message after your service restarts, how would you prevent the recipient from receiving the payment twice?”
- “How would you design the system so that if a service restarts and an external service resends the same 50,000-peso credit message, the customer is not credited twice? How would you handle a late event indicating that the transaction is already settled or in an intermediate state?”
What are SQL joins, and how do inner, left outer, and full outer joins differ?
Asked in 11 of 587 interviews · last asked Oct 22 · Concept · Junior
Show answer
SQL joins combine rows from tables based on a join condition. INNER JOIN returns only matching rows. LEFT OUTER JOIN also keeps unmatched rows from the left table, with NULLs for right-side columns. FULL OUTER JOIN returns matching rows plus unmatched rows from both tables, with NULLs for columns from the missing side.
- Use INNER JOIN when you need only rows that match.
- Use LEFT JOIN when every left-side row must stay, even without a match.
- Use FULL OUTER JOIN to keep matches and find unmatched rows on either side.
- A row with multiple matches produces multiple result rows.
Code
```
Join on ID. Left IDs: 1, 2. Right IDs: 2, 3.
Results shown as (left ID, right ID):
INNER JOIN:      (2, 2)
LEFT OUTER JOIN: (1, NULL), (2, 2)
FULL OUTER JOIN: (1, NULL), (2, 2), (NULL, 3)
```
What the interviewer checks: The interviewer is checking whether you know which rows each join keeps and where NULLs appear.
Common mistake (in 3 of 7 reviewed answers): Many answers forget that FULL OUTER JOIN keeps matching rows too, so explain both matches and unmatched rows.
Other ways it was asked
- “What is the difference between INNER JOIN, LEFT JOIN, and FULL OUTER JOIN?”
- “What’s the difference between an inner and outer join?”
- “What’s the difference between INNER JOIN, LEFT JOIN, and FULL OUTER JOIN?”
What are JWTs, what types are there, and how are they used for authentication?
Asked in 11 of 587 interviews · last asked Oct 15 · Concept · Junior
Show answer
JWTs carry claims, such as a user ID and expiry time. They can be signed (JWS) or encrypted (JWE). After login, the server issues a token, which the client sends with later requests. For signed tokens, the server verifies the signature, expiry, issuer, and audience before trusting the claims.
- Signed JWTs are readable, not secret. Send tokens over HTTPS, commonly in the Authorization: Bearer header.
- Access tokens authorize API requests. Refresh tokens obtain new access tokens; neither must be a JWT.
- Unsigned JWTs exist, but should not be accepted as authentication tokens.
- Logout does not automatically invalidate a JWT. Use short expiry times and a revocation strategy when needed.
What the interviewer checks: The interviewer checks whether you understand token formats, the login flow, and safe server-side validation.
Common mistake (in 5 of 5 reviewed answers): Many answers forget that JWT authentication includes issuing a token after login, sending it with requests, and validating it on the server.
Other ways it was asked
- “What is JWT?”
- “What is JWT-based authentication?”
- “Can you explain JWT authentication?”
Can you explain the SOLID principles?
Asked in 10 of 587 interviews · last asked Oct 15 · Concept · Mid-level
Show answer
SOLID names five principles for object-oriented design. Single Responsibility means one reason to change; Open/Closed means open for extension, closed for modification. Liskov Substitution means subtypes must preserve the base type’s expected behavior; Interface Segregation means clients should not depend on methods they do not need. Dependency Inversion means high-level and low-level code depend on abstractions.
- Single Responsibility does not mean a class can have only one method.
- Open/Closed favors adding new behavior without changing stable code.
- Liskov Substitution requires subtypes to honor the base type’s contract, not just match its methods.
- Dependency Inversion keeps business rules independent of details such as database implementations.
What the interviewer checks: The interviewer checks whether you understand all five principles and how they make code easier to change.
Other ways it was asked
- “What are the SOLID principles?”
- “What are the SOLID principles? Can you name three or all five?”
- “What is your understanding of the SOLID principles in software development?”
What is the N+1 query problem, and how can you avoid it?
Asked in 9 of 587 interviews · last asked Oct 15 · Concept · Mid-level
Show answer
The N+1 query problem happens when one query loads a list, then a separate query loads related data for each item. This causes many database round trips and can slow requests. Avoid it by fetching related data together, using joins, eager loading, or batch queries.
- Lazy loading inside a loop is a common cause.
- Eager loading may use joins or separate batch queries, depending on the ORM.
- Joins can repeat parent data when loading many related rows.
- Check query logs to confirm that query count does not grow with each item.
What the interviewer checks: The interviewer checks whether you can spot repeated database calls and choose an efficient way to load related data.
Other ways it was asked
- “What is the N+1 query problem?”
- “What is the N+1 query problem, and how do you fix it?”
- “Explain the N+1 query problem, how to detect it, and how to resolve it.”
What is a HashMap, and how does it work internally in Java?
Asked in 8 of 587 interviews · last asked Oct 16 · Concept · Senior
Show answer
A HashMap stores key-value pairs. Java mixes the key’s hashCode() result and uses it to select a bucket. It checks keys with equals() to find the matching entry. Different keys can share a bucket; entries form a linked list, which can become a tree in Java 8 and later when size conditions are met.
- get() and put() take O(1) time on average when hashes spread keys well.
- The bucket array grows when the entry count exceeds the capacity times the load factor.
- Equal keys must have equal hash codes; avoid changing key fields used by equals() or hashCode().
- HashMap allows one null key and null values, but is not thread-safe.
What the interviewer checks: The interviewer checks whether you understand bucket selection, key matching, collisions, and basic performance.
Common mistake (in 5 of 5 reviewed answers): Many answers forget that Java mixes the key’s hash code and uses the result to select a bucket.
Other ways it was asked
- “What is a HashMap?”
- “How does Java's HashMap work?”
- “How does HashMap work in Java?”
What is Python?
Asked in 8 of 587 interviews · last asked Sep 27 · Concept · Junior
Show answer
Python is a high-level, general-purpose programming language with clear, readable syntax. It is dynamically typed, so types belong to values rather than being fixed for variable names. It supports several programming styles and is widely used for backend services, automation, and data analysis.
- High-level means it handles many low-level details, such as memory management.
- Dynamic typing means type checks happen at runtime.
- It supports procedural, object-oriented, and functional programming styles.
- Its large standard library and package ecosystem help with common development tasks.
What the interviewer checks: The interviewer is checking whether you can clearly explain Python’s main features and common uses.
Other ways it was asked
- “What is the definition of Python?”
What is middleware, and how is it used in ASP.NET Core?
Asked in 8 of 587 interviews · last asked Oct 6 · Concept · Junior
Show answer
Middleware is a component in the ASP.NET Core HTTP request pipeline. It can handle a request, pass it to the next component, and process the response on the way back. I configure middleware in Program.cs for tasks such as error handling, routing, authentication, and authorization. The order matters.
- Requests move through middleware in registration order; response processing runs in reverse order.
- Middleware can short-circuit the pipeline by not calling the next component.
- Place authentication before authorization so access checks can use the user's identity.
- Use app.Use for middleware registration, app.Run for terminal middleware, and app.Map to branch by path.
What the interviewer checks: The interviewer is checking whether you understand pipeline order, request and response flow, and short-circuiting.
Other ways it was asked
- “What is middleware?”
- “What is middleware, and how is it used in an application?”
- “What is middleware in ASP.NET?”
What is the difference between mutable and immutable objects?
Asked in 8 of 587 interviews · last asked Oct 22 · Concept · Junior
Show answer
A mutable object can change after it is created. An immutable object cannot change; an operation that changes its value must produce another object. Mutating an object keeps its identity, while assigning a different object changes what a variable refers to. Memory allocation details depend on the language and runtime.
- In Python, lists are mutable, while strings and integers are immutable.
- Changes to a mutable object are visible through all references to that object.
- Immutability does not stop a variable from being assigned a different object.
Python
```
items = [1, 2]
shared = items
items.append(3)
print(shared)  # [1, 2, 3]

name = "Sam"
original = name
name = name + "!"
print(original)  # Sam
print(name)      # Sam!
```
What the interviewer checks: The interviewer is checking whether you understand object changes, shared references, and variable reassignment.
Other ways it was asked
- “Can you explain mutable and immutable objects, with examples?”
- “What is the difference between mutable and immutable objects in terms of memory allocation?”
- “What are mutable and immutable objects?”

Backend programming languages and frameworks interview questions

Language and framework questions cover types, collections, memory management, exceptions, and backend language basics. 714 questions.

What is exception handling, and how are exceptions handled in Java?
Asked in 8 of 587 interviews · last asked Oct 5 · Concept · Junior
Show answer
Exception handling deals with events that interrupt normal program flow. In Java, use try for code that may fail, catch to handle exceptions, and finally for cleanup. Use throw to raise an exception and throws to declare exceptions a method may pass to its caller.
- Checked exceptions must be caught or declared; unchecked exceptions do not have this requirement.
- Catch specific exception types before broader types.
- Use try-with-resources to close resources automatically.
- A finally block normally runs even after return, but not if the JVM terminates.
What the interviewer checks: The interviewer checks whether you understand Java exception handling, checked exceptions, and safe resource cleanup.
Other ways it was asked
- “What is exception handling?”
- “How do you handle exceptions in Java?”
- “What is exception handling in Java?”
What string hashing algorithm is used in Python?
Asked in 7 of 587 interviews · last asked Oct 20 · Concept · Mid-level
Other ways it was asked
- “What hashing algorithm is used for strings in Python?”
What is the difference between StringBuffer and StringBuilder?
Asked in 6 of 587 interviews · last asked Oct 13 · Concept · Junior
Other ways it was asked
- “What is the difference between String, StringBuilder, and StringBuffer?”
- “What is one key difference between StringBuilder and StringBuffer in Java, and when would you use each?”
- “What is the difference between String and StringBuffer?”
Can you explain Spring Boot?
Asked in 6 of 587 interviews · last asked Oct 1 · Concept · Junior
Other ways it was asked
- “What is Spring Boot?”
- “What is Spring Boot? Can you give me an example?”
- “What is a Spring Boot application?”
What is the difference between Spring Framework and Spring Boot?
Asked in 6 of 587 interviews · last asked Oct 16 · Concept · Junior
Other ways it was asked
- “What is the difference between Spring and Spring Boot?”
- “What is the difference between the Spring Framework and Spring Boot?”
- “How does Spring Boot differ from Spring, and what benefits does Spring Boot provide?”
What are the different types of bean scopes in Spring?
Asked in 5 of 587 interviews · last asked Sep 28 · Concept · Mid-level
Other ways it was asked
- “What different scopes can a Spring bean have?”
- “What scopes are available for Spring beans, and what does each scope mean?”
- “What bean scopes are available in Spring?”
What is the difference between `==` and `.equals()` in Java?
Asked in 5 of 587 interviews · last asked Oct 8 · Concept · Junior
Other ways it was asked
- “What is the difference between `==` and `.equals()` in core Java?”
- “What is the difference between == and the equals method?”
- “What is the difference between == and .equals()?”
What is the difference between an array and an ArrayList?
Asked in 5 of 587 interviews · last asked Oct 22 · Concept · Junior
Other ways it was asked
- “What is the difference between an array and a list?”
- “What is the difference between ArrayList and List in Java?”
- “How is an array different from an ArrayList?”
What are decorators in Python?
Asked in 5 of 587 interviews · last asked Oct 15 · Concept · Junior
Other ways it was asked
- “When would you use Python decorators?”
- “What is a decorator in Python?”
- “What is a Python decorator?”
What is Java?
Asked in 5 of 587 interviews · last asked Sep 27 · Concept · Junior
What is garbage collection in Java?
Asked in 5 of 587 interviews · last asked Jun 21 · Concept · Junior
Other ways it was asked
- “What is garbage collection, and how can it affect the performance of a Java application?”
- “Can you explain garbage collection?”
- “What is garbage collection?”
What is the difference between IEnumerable and IQueryable in .NET?
Asked in 4 of 587 interviews · last asked Oct 2 · Concept · Mid-level
Other ways it was asked
- “What is the difference between IQueryable and IEnumerable?”
- “In the .NET framework, what’s the difference between IEnumerable and IQueryable, and when would you choose one over the other?”
- “What is the difference between IEnumerable and IQueryable, especially when working with a database?”

Show 3 more

How would you design an immutable class in Java?
Asked in 4 of 587 interviews · last asked Oct 6 · Concept · Mid-level
Other ways it was asked
- “How do you make a class immutable in Java?”
- “How can you make a class immutable?”
- “How can you create an immutable class in Java?”
Can you tell me the difference between a shallow copy and a deep copy in Python?
Asked in 4 of 587 interviews · last asked Oct 7 · Concept · Mid-level
Other ways it was asked
- “What is the difference between a deep copy and a shallow copy?”
- “What is the difference between shallow and deep copying in Python, and when can a shallow copy cause problems?”
- “What is the difference between a shallow copy and a deep copy in Python?”
What is the difference between Java and JavaScript?
Asked in 4 of 587 interviews · last asked Jul 2 · Concept · Junior
Other ways it was asked
- “What is the difference between Java and JavaScript, and when would you use each?”
- “What is the difference between JavaScript and Java?”

OOP and design patterns interview questions

Explore OOP principles, SOLID, dependency injection, and design patterns for code that is easier to maintain. 239 questions.

What is inheritance, and what types of inheritance does Java support?
Asked in 8 of 587 interviews · last asked Sep 27 · Concept · Junior
Other ways it was asked
- “What is inheritance?”
- “Which types of inheritance does Java support?”
- “What is the concept of inheritance?”
What are design patterns, and can you describe examples and when you would use them?
Asked in 8 of 587 interviews · last asked Oct 15 · Concept · Mid-level
Other ways it was asked
- “Can you name some design patterns you have worked with?”
- “When would you use design patterns in software development?”
- “What are some common software design patterns, and when would you use them?”
What is abstraction?
Asked in 6 of 587 interviews · last asked Sep 22 · Concept · Junior
Can you describe the difference between transient, scoped, and singleton dependency injection lifetimes in .NET?
Asked in 5 of 587 interviews · last asked Sep 10 · Concept · Mid-level
Other ways it was asked
- “Can you explain the differences between transient, scoped, and singleton service lifetimes in ASP.NET Core, and give a concrete example of when you would use each?”
- “In ASP.NET Core, how do you decide between singleton, scoped, and transient? What is a real example of a bug caused by choosing the wrong lifetime?”
- “What is the difference between scoped and transient service lifetimes in .NET dependency injection?”
What is an interface in Java?
Asked in 5 of 587 interviews · last asked Sep 30 · Concept · Junior
Other ways it was asked
- “What is an interface in Java, and what is it used for?”
- “What advantages do interfaces provide in Java?”
What is inversion of control?
Asked in 5 of 587 interviews · last asked Oct 7 · Concept · Junior
Other ways it was asked
- “What is inversion of control (IoC)?”
- “What does inversion of control mean in a Spring application?”
- “What is inversion of control in Spring Boot?”
Can you explain how Java handles multiple inheritance?
Asked in 4 of 587 interviews · last asked Oct 8 · Concept · Junior
Other ways it was asked
- “How can you implement multiple inheritance in Java?”
- “How can you achieve multiple inheritance in Java when classes do not support it?”
- “Why does Java not allow multiple inheritance?”
What are the types of dependency injection in Spring?
Asked in 4 of 587 interviews · last asked Oct 16 · Concept · Junior
Other ways it was asked
- “What types of dependency injection does Spring support, which type is recommended, and why?”
- “What are the different ways of dependency injection in Spring?”
- “What are the three ways to inject dependencies in Spring?”
Can a static method be overridden?
Asked in 3 of 587 interviews · last asked Sep 9 · Concept · Junior
Other ways it was asked
- “Can you override a static method in Java?”
- “Why can't static methods be overridden in Java?”
What is the Singleton design pattern?
Asked in 3 of 587 interviews · last asked Sep 9 · Concept · Junior
Other ways it was asked
- “What is the singleton pattern?”
- “What is the singleton design pattern, and when would you use it?”
What are the main categories of software design patterns?
Asked in 2 of 587 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 587 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.”

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What is a captive dependency in .NET dependency injection, and why can it cause problems?
Asked in 2 of 587 interviews · last asked Sep 29 · Concept · Mid-level
Other ways it was asked
- “What is a captive dependency in dependency injection?”
What makes code maintainable?
Asked in 2 of 587 interviews · last asked Sep 9 · Concept · Mid-level
Other ways it was asked
- “How do you make sure the code you write is maintainable for others?”

Backend coding and data structures interview questions

Coding tasks cover strings, arrays, linked lists, trees, graphs, sorting, searching, and complexity analysis. 377 questions.

Write code to reverse a string.
Asked in 5 of 587 interviews · last asked Aug 14 · Write code · Junior
Other ways it was asked
- “Write Java code to reverse a string.”
- “How would you write code to reverse a string?”
- “How would you reverse a string?”
What is a list?
Asked in 3 of 587 interviews · last asked Oct 17 · Concept · Junior
Other ways it was asked
- “What is a list, and what types of items can it contain?”
What is an array?
Asked in 3 of 587 interviews · last asked Oct 22 · Concept · Junior
Other ways it was asked
- “Define an array.”
- “What is an array in JavaScript?”
What is a linked list, and how does it work?
Asked in 3 of 587 interviews · last asked Sep 13 · Concept · Junior
Other ways it was asked
- “Can you explain a linked list?”
- “How does a linked list work?”
Write code to find the largest number in an array.
Asked in 3 of 587 interviews · last asked Oct 22 · Write code · Junior
Other ways it was asked
- “Write a simple function to find the largest number in an array of numbers.”
- “Write code to find the maximum number in an array in O(n) time.”
Write code for the Two Sum problem.
Asked in 2 of 587 interviews · last asked Oct 19 · Write code · Junior
Other ways it was asked
- “Can you write the code for the Two Sum problem?”
- “Can you write the solution for Two Sum?”
- “How would you solve the Two Sum coding question?”
Find the longest substring without repeated characters.
Asked in 2 of 587 interviews · last asked Oct 5 · Write code · Junior
Other ways it was asked
- “Given a string, find the length of the longest substring without repeating characters.”
- “How can you find the length of the longest substring without repeating characters without repeatedly scanning the substring?”
What is Dijkstra's algorithm, and when is it used?
Asked in 2 of 587 interviews · last asked Oct 9 · Concept · Junior
Other ways it was asked
- “What is the Dijkstra algorithm?”
What is the time complexity of merge sort and quicksort?
Asked in 2 of 587 interviews · last asked Oct 9 · Concept · Junior
Other ways it was asked
- “What are the time complexities of bubble sort and merge sort?”
- “What is the time complexity of merge sort?”
Given existing conference room bookings as an array of start- and end-time pairs and a new meeting request in the same format, write a function that determines whether the new meeting can be booked without overlapping any existing meeting.
Asked in 2 of 587 interviews · last asked Jun 23 · Write code · Junior
Other ways it was asked
- “Given a list of existing conference-room bookings, each represented by a start time and an end time, and one new meeting request in the same format, write a function that returns true if the new meeting can be booked without overlapping any existing meeting, and false otherwise. Assume one meeting ending exactly when another starts is allowed.”
Why would you choose breadth-first search (BFS) over depth-first search (DFS)?
Asked in 2 of 587 interviews · last asked Sep 4 · Concept · Mid-level
Other ways it was asked
- “What is the difference between depth-first search (DFS) and breadth-first search (BFS)?”
How would you design and implement an LRU cache?
Asked in 2 of 587 interviews · last asked Jul 27 · Write code · Mid-level

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Write a function that prints “even” if a number x is even, or “odd” otherwise.
Asked in 2 of 587 interviews · last asked Aug 21 · Write code · Junior
Other ways it was asked
- “Write a program that prints "even" if a number x is even, otherwise prints "odd".”
Design an LRU cache with get(key) and put(key, value) operations that run in O(1) time.
Asked in 2 of 587 interviews · last asked Sep 10 · Write code · Junior
Other ways it was asked
- “Design and implement an LRU cache with get(key) and put(key, value) operations.”
Write code to check whether parentheses are valid.
Asked in 2 of 587 interviews · last asked Oct 20 · Write code · Junior
Other ways it was asked
- “What test cases would you write to ensure a function that checks whether parentheses are valid, including nested parentheses, is correct and robust?”
- “How would you structure a function that checks whether parentheses are valid, and what should it do when it encounters a closing bracket that does not match the expected type?”
What should the maximum subarray sum be when all the numbers are negative?
Asked in 2 of 587 interviews · last asked Sep 10 · Concept · Mid-level
Other ways it was asked
- “How would you find the maximum-sum contiguous subarray when the array may contain negative numbers?”
In a HashMap, what exactly happens during a resize or rehash, how are buckets redistributed, and what determines which bucket an existing entry moves to?
Asked in 2 of 587 interviews · last asked Oct 17 · Concept · Senior
Other ways it was asked
- “In Java, what happens during HashMap resizing and rehashing? How are entries redistributed across the new table without recomputing every hash from scratch?”
How would you check whether a string is a palindrome?
Asked in 2 of 587 interviews · last asked Jul 10 · Write code · Junior
Other ways it was asked
- “How can you identify whether a word is a palindrome?”
Can you explain the difference between a stack and a queue, and give a real-world example of each?
Asked in 2 of 587 interviews · last asked Aug 14 · Concept · Junior
Other ways it was asked
- “What's the difference between a stack and a queue, and when would you choose each?”

All Data Structures and Algorithms interview questions

Multithreading and async programming interview questions

Compare processes, threads, and async execution, with questions on scheduling, synchronization, deadlocks, and safe concurrent data processing. 256 questions.

How do HashMap and ConcurrentHashMap differ in thread safety and concurrent reads and updates?
Asked in 7 of 587 interviews · last asked Oct 23 · Concept · Mid-level
Other ways it was asked
- “How is HashMap different from ConcurrentHashMap in Java?”
- “Can you explain the difference between HashMap and ConcurrentHashMap?”
- “Can you explain the difference between HashMap and ConcurrentHashMap, and when would you use each?”
How do async and await work in C#, and when would you use them?
Asked in 6 of 587 interviews · last asked Oct 5 · Concept · Mid-level
Other ways it was asked
- “What does async/await do in C#, and what benefits does it provide?”
- “What are async/await operations in C#, and how do they work?”
- “How does asynchronous programming work in .NET and C#, and when would you use it?”
Can you explain what the GIL is and how it affects Python programs?
Asked in 4 of 587 interviews · last asked Jul 16 · Concept · Mid-level
Other ways it was asked
- “What is the GIL (Global Interpreter Lock) in CPython, and how does it affect multithreading?”
- “What is the GIL in Python?”
- “In Python 3, what is the GIL, and what kinds of workloads does it prevent Python threads from scaling on?”
What is the role of the volatile keyword in Java?
Asked in 4 of 587 interviews · last asked Oct 16 · Concept · Mid-level
Other ways it was asked
- “What does volatile do in Java?”
- “Can you tell me about the volatile keyword?”
What is multithreading?
Asked in 4 of 587 interviews · last asked Sep 4 · Concept · Junior
Other ways it was asked
- “What is multithreading in Java, and when would you use it?”
- “What is multithreading, and when is it useful?”
What is the event loop structure of Node.js?
Asked in 3 of 587 interviews · last asked Sep 18 · Concept · Mid-level
Other ways it was asked
- “Can you explain how the event loop works in Node.js?”
- “What is the role of the event loop in Node.js?”
- “Can you explain the Node.js event-loop phases?”
What problems can arise when multiple threads share data, and how can you prevent or handle them?
Asked in 3 of 587 interviews · last asked Oct 14 · Scenario · Mid-level
Other ways it was asked
- “In a multithreaded program, what kinds of issues can occur when threads share data, and how would you prevent them?”
- “What problems can occur when multiple threads access shared data, and how would you prevent them?”
- “What is the difference between a process and a thread, and what kinds of problems can arise when multiple threads share data?”
In Java, what's the difference between volatile and synchronized in terms of visibility, atomicity, and ordering?
Asked in 3 of 587 interviews · last asked Oct 17 · Concept · Senior
Other ways it was asked
- “In Java, what are the key differences between volatile, synchronized, and AtomicInteger regarding visibility, atomicity, and ordering guarantees?”
- “In Java, what's the difference between volatile and synchronized regarding visibility and atomicity, and when would you choose one over the other?”
What does ConfigureAwait do, and when might you use ConfigureAwait(false)?
Asked in 3 of 587 interviews · last asked Sep 29 · Concept · Senior
Other ways it was asked
- “What is ConfigureAwait(false) and when would you use it?”
- “In ASP.NET .NET Framework, what does ConfigureAwait(false) change about continuation scheduling, and when would you avoid using it?”
What is Task.Run and how is it different from async/await?
Asked in 3 of 587 interviews · last asked Oct 1 · Concept · Mid-level
Other ways it was asked
- “When would you use Task.Run, and when would you use async and await directly? What is the key difference, particularly for an I/O-bound operation such as a network call?”
- “What is the difference between Task.Run and standard synchronous execution in .NET?”
What is a channel in Go?
Asked in 3 of 587 interviews · last asked Oct 13 · Concept · Junior
Other ways it was asked
- “What is a channel in Go, and how is it used?”
- “What types of channels do we have in Go?”
What is a synchronized block in Java?
Asked in 3 of 587 interviews · last asked Sep 11 · Concept · Junior
Other ways it was asked
- “What does a synchronized block or method lock in Java?”
- “What is the difference between a synchronized method and a synchronized block in Java?”

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Can you describe a scenario where using Task.Run is an anti-pattern in server code, and explain what you would do instead?
Asked in 2 of 587 interviews · last asked May 16 · Scenario · Senior
Other ways it was asked
- “Can you describe a scenario where using Task.Run is an anti-pattern in server code?”
- “What should you use instead of Task.Run in server code when it is an anti-pattern, and why?”
- “In what server-code scenarios is Task.Run an anti-pattern, and what problems can it cause?”
What is the difference between debouncing and throttling?
Asked in 2 of 587 interviews · last asked Oct 7 · Concept · Junior
How does asynchronous programming work in Python?
Asked in 2 of 587 interviews · last asked Sep 16 · Concept · Mid-level
Other ways it was asked
- “What is asynchronous programming in Python, and why is it useful?”
- “Does asynchronous programming in Python execute tasks simultaneously?”
Is JavaScript single-threaded or multithreaded, and how does it handle asynchronous work?
Asked in 2 of 587 interviews · last asked Aug 25 · Concept · Mid-level
Other ways it was asked
- “Is JavaScript single-threaded or multithreaded?”
How does PHP handle concurrency?
Asked in 2 of 587 interviews · last asked Jul 27 · Concept · Senior
Other ways it was asked
- “Can you give an example of how concurrency can be handled in PHP?”
- “How does PHP handle concurrency when multiple tasks need to run at once?”
How do you manage concurrency and threads in Java?
Asked in 2 of 587 interviews · last asked Jun 15 · Concept · Senior
Other ways it was asked
- “Can you explain concurrency and thread management in Java?”
What is the GIL, and how does it work?
Asked in 2 of 587 interviews · last asked Oct 15 · Concept · Mid-level
Other ways it was asked
- “What is the GIL?”

SQL and database interview questions

SQL and database questions cover joins, schemas, indexes, transactions, locking, query tuning, ORM behavior, and database choices. 527 questions.

How would you optimize a slow database query?
Asked in 8 of 587 interviews · last asked Oct 20 · Scenario · Mid-level
Other ways it was asked
- “How would you optimize a slow query?”
- “How can you optimize a slow SQL query?”
- “How would you optimize a slow SQL query?”
Write a SQL query to find the second-highest salary.
Asked in 6 of 587 interviews · last asked Oct 22 · Write code · Junior
Other ways it was asked
- “Write an SQL query to find the third-highest employee salary.”
- “Write a MySQL query to get the second highest salary from an employee table with only one salary column.”
- “Write a SQL query to find the second-highest salary in each department.”
What's the difference between WHERE and HAVING?
Asked in 5 of 587 interviews · last asked Oct 13 · Concept · Junior
Other ways it was asked
- “What is the difference between WHERE and HAVING?”
- “What is the difference between WHERE and HAVING clauses in SQL?”
- “What is the difference between WHERE and HAVING in SQL?”
What does the ACID acronym stand for in database transactions?
Asked in 5 of 587 interviews · last asked Sep 3 · Concept · Junior
Other ways it was asked
- “What does ACID mean in the context of databases?”
- “What are the ACID properties of a database transaction?”
- “What are the ACID properties in database transactions?”
What is the difference between SQL and NoSQL databases?
Asked in 5 of 587 interviews · last asked Oct 23 · Concept · Junior
Other ways it was asked
- “What is the key difference between SQL and NoSQL databases?”
- “What is the main difference between a SQL and NoSQL database?”
- “What is the difference between SQL and NoSQL?”
What is the difference between a primary key and a unique constraint?
Asked in 4 of 587 interviews · last asked Oct 16 · Concept · Junior
Other ways it was asked
- “What is the difference between a primary key and a unique key?”
- “What is the difference between a unique key and a primary key?”
- “In a relational database, what's the difference between a primary key and a unique constraint, and when would you use each?”
What is the difference between DELETE, TRUNCATE, and DROP?
Asked in 4 of 587 interviews · last asked Oct 16 · Concept · Junior
Other ways it was asked
- “Could you explain the difference between DELETE, TRUNCATE, and DROP in SQL?”
- “What is the difference between TRUNCATE and DELETE?”
- “What is the difference between DROP and TRUNCATE?”
What are stored procedures, and when would you use them?
Asked in 4 of 587 interviews · last asked Oct 16 · Concept · Junior
Other ways it was asked
- “What are stored procedures?”
- “What is a stored procedure in MySQL, and when would you use one?”
How do you determine which database columns should be indexed?
Asked in 3 of 587 interviews · last asked Sep 28 · Scenario · Mid-level
Other ways it was asked
- “Which database columns would you index, and how would you decide?”
- “How would you choose which columns to index in a database?”
What is the difference between pessimistic locking and optimistic locking?
Asked in 3 of 587 interviews · last asked Oct 12 · Concept · Mid-level
Other ways it was asked
- “What is the difference between pessimistic and optimistic locking?”
- “How do optimistic and pessimistic locking work?”
What are SQL execution plans, and how can you use them to investigate query performance?
Asked in 3 of 587 interviews · last asked Oct 22 · Concept · Mid-level
Other ways it was asked
- “What is a SQL execution plan, and how do you use it to investigate query performance?”
- “How do you use SQL execution plans to diagnose and optimize queries?”
- “How do you read a SQL execution plan, and what information can it provide?”
How would you investigate a problem with SQL or a database?
Asked in 3 of 587 interviews · last asked Jun 18 · Scenario · Mid-level
Other ways it was asked
- “How would you investigate a SQL performance problem?”
- “How would you investigate a database issue that is causing an application problem?”

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In Spring, does @Transactional work when one method calls another @Transactional method in the same class?
Asked in 3 of 587 interviews · last asked Sep 1 · Concept · Senior
Other ways it was asked
- “If a method annotated with @Transactional is called from another method in the same class, will a transaction start?”
- “What happens in Spring when one transactional method calls another transactional method?”
What is a database deadlock, and why does it occur?
Asked in 3 of 587 interviews · last asked Oct 2 · Concept · Mid-level
Other ways it was asked
- “What is a database deadlock?”
What are database triggers, and when would you use them?
Asked in 2 of 587 interviews · last asked Sep 4 · Concept · Mid-level
Other ways it was asked
- “What are some use cases for database triggers?”
- “What is a database trigger, and when would you use one?”
What are the disadvantages of database indexes?
Asked in 2 of 587 interviews · last asked Sep 21 · Concept · Mid-level
Other ways it was asked
- “What are the disadvantages of using an index?”
What is the difference between MongoDB and a SQL database?
Asked in 2 of 587 interviews · last asked Oct 16 · Concept · Junior
Other ways it was asked
- “What is the difference between SQL and MongoDB, and when would you prefer MongoDB?”

All SQL interview questions

REST API and backend security interview questions

Review HTTP, API design, pagination, integrations, authentication, authorization, tokens, input checks, and safe data handling. 459 questions.

What is the difference between the HTTP PUT and PATCH methods?
Asked in 8 of 587 interviews · last asked Oct 23 · Concept · Junior
Other ways it was asked
- “What is the difference between PUT and PATCH?”
- “When would you use the PUT method, and when would you use PATCH?”
- “What is the difference between POST and PATCH?”
What is idempotency, and why is it useful for HTTP methods and retries?
Asked in 8 of 587 interviews · last asked Oct 16 · Concept · Mid-level
Other ways it was asked
- “Which HTTP methods are idempotent?”
- “What is idempotency?”
- “What is idempotency, and where is it useful in a software system?”
What HTTP methods are commonly used in REST APIs, and what is each used for?
Asked in 7 of 587 interviews · last asked Oct 23 · Concept · Junior
Other ways it was asked
- “What are the main HTTP verbs commonly used in APIs?”
- “What HTTP methods are available in an API?”
- “Can you name some common HTTP methods?”
How do authentication and authorization differ, and how do JWTs support them in ASP.NET Core?
Asked in 7 of 587 interviews · last asked Oct 23 · Concept · Mid-level
Other ways it was asked
- “What is the difference between authentication and authorization?”
- “What is the difference between authentication and authorization, and how do you manage authorization for your services?”
- “How do authentication and authorization work?”
What is a REST API?
Asked in 6 of 587 interviews · last asked Oct 5 · Concept · Junior
Other ways it was asked
- “What are REST APIs?”
- “What do you mean by a RESTful API?”
- “What is a REST API, and how would you implement one?”
How do you secure a REST API?
Asked in 5 of 587 interviews · last asked Oct 16 · Scenario · Senior
Other ways it was asked
- “How do you secure a web API?”
- “What mechanisms would you use to secure a REST API?”
- “How would you secure an API and protect the data it handles?”
What do you consider when designing a REST API?
Asked in 5 of 587 interviews · last asked Oct 16 · Concept · Mid-level
Other ways it was asked
- “What are the best practices for designing a REST API?”
- “What should you consider when designing an API?”
- “How do you design REST APIs and their architecture?”
What is a SQL injection vulnerability? What is its impact, and how can it be prevented?
Asked in 4 of 587 interviews · last asked Oct 17 · Concept · Mid-level
Other ways it was asked
- “What is SQL injection, how can an attacker exploit it, and how can it be prevented?”
- “What is a SQL injection vulnerability? What is the impact, and how can it be prevented?”
- “What is SQL injection?”
How is GraphQL different from REST?
Asked in 3 of 587 interviews · last asked Oct 22 · Concept · Mid-level
Other ways it was asked
- “What is the difference between GraphQL and REST APIs?”
- “What is GraphQL and how does it differ from REST? What problems does it solve?”
- “What security and performance differences should you consider when comparing GraphQL and REST APIs?”
What is the difference between SOAP and REST?
Asked in 3 of 587 interviews · last asked Oct 15 · Concept · Junior
Other ways it was asked
- “What is the difference between REST and SOAP?”
- “How do SOAP and REST differ, and in what situations would you choose each?”
How do offset pagination and cursor pagination differ?
Asked in 3 of 587 interviews · last asked Oct 14 · Concept · Mid-level
Other ways it was asked
- “What is the difference between using an offset and a cursor for database pagination?”
- “Explain the trade-offs between offset pagination and cursor pagination.”
What is the difference between PUT and POST?
Asked in 3 of 587 interviews · last asked Oct 13 · Concept · Junior
Other ways it was asked
- “What is the difference between POST and GET?”
- “What is the difference between the PUT and POST methods in Spring Boot?”

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What's the difference between encryption and hashing?
Asked in 2 of 587 interviews · last asked Sep 21 · Concept · Junior
Other ways it was asked
- “How is hashing different from encryption?”
What is the difference between REST APIs and GraphQL, how is GraphQL implemented under the hood, and when should each be used?
Asked in 2 of 587 interviews · last asked Oct 23 · Concept · Senior
Other ways it was asked
- “When would you use a REST API versus GraphQL, and how do they differ?”
What is CORS, and how does it affect cross-origin requests?
Asked in 2 of 587 interviews · last asked Apr 15 · Concept · Mid-level
Other ways it was asked
- “What is CORS?”
- “What is CORS, and how does it work?”
How do you ensure your API versioning doesn't disrupt existing clients, especially when making significant updates?
Asked in 2 of 587 interviews · last asked Oct 23 · Scenario · Mid-level
Other ways it was asked
- “How do you version your API?”
How would you implement authentication in a web application?
Asked in 2 of 587 interviews · last asked Sep 29 · Scenario · Mid-level
Other ways it was asked
- “Suppose you are building a web application: how do you handle authentication?”
- “How would you handle authorization in a web application?”
What security measures do you follow when creating infrastructure and securing applications?
Asked in 2 of 587 interviews · last asked Oct 20 · Scenario · Mid-level
Other ways it was asked
- “What security practices do you apply when developing applications?”
- “What security considerations do you account for in an application's codebase and infrastructure?”
What are WebSockets, and when would you use them?
Asked in 2 of 587 interviews · last asked Mar 5 · Concept · Mid-level
Other ways it was asked
- “What are WebSockets?”

Backend system design and microservices interview questions

System design questions explore scaling, caching, messaging, distributed consistency, reliable payments, batch processing, and AI systems. 660 questions.

How do microservices communicate with each other?
Asked in 7 of 587 interviews · last asked Oct 23 · Concept · Mid-level
Other ways it was asked
- “What are the ways microservices can communicate with each other?”
- “How does communication between two microservices happen?”
- “How would you communicate between two microservices?”
What is the difference between horizontal and vertical scaling, and when would you use each?
Asked in 5 of 587 interviews · last asked Oct 22 · Concept · Mid-level
Other ways it was asked
- “What is the difference between horizontal and vertical scaling?”
- “What is the difference between vertical and horizontal scaling?”
- “What is horizontal scaling, and when would you use it?”
What is the Saga pattern in microservices, and when would you use it?
Asked in 5 of 587 interviews · last asked Oct 16 · Concept · Senior
Other ways it was asked
- “What is the Saga design pattern in microservices, and when would you use it?”
- “What is the core idea behind the Saga pattern in microservices architecture?”
- “What is the saga pattern?”
How do you design and build microservices?
Asked in 4 of 587 interviews · last asked Jul 31 · Design · Mid-level
Other ways it was asked
- “What are some key factors to consider when designing scalable microservices?”
- “How do you design and implement microservices?”
- “What architectural patterns can you use to design microservices?”
How would you implement a retrieval-augmented generation (RAG) system?
Asked in 3 of 587 interviews · last asked Sep 10 · Design · Mid-level
Other ways it was asked
- “Can you explain retrieval-augmented generation (RAG) in simpler terms?”
- “What is retrieval-augmented generation (RAG), and how is it used in data-related projects?”
- “Where could you use retrieval-augmented generation (RAG) in an application architecture?”
What are the differences between Kafka and RabbitMQ?
Asked in 3 of 587 interviews · last asked Jun 24 · Concept · Mid-level
Other ways it was asked
- “What is the difference between Kafka and RabbitMQ?”
- “What are the implementation differences between Kafka and RabbitMQ, and how do you decide which one to use?”
What is Redis, and how would you use it in an application?
Asked in 3 of 587 interviews · last asked Oct 12 · Tooling · Junior
Other ways it was asked
- “How can Redis be used in an application?”
- “What is Redis, and why do we need to use it in our application?”
How would you design a multi-tenant ERP architecture?
Asked in 3 of 587 interviews · last asked Oct 7 · Design · Senior
Other ways it was asked
- “How would you design a multi-tenant architecture?”
- “Explain your multi-tenant ERP architecture.”
How do you design a rate limiter?
Asked in 3 of 587 interviews · last asked Sep 6 · Design · Senior
Other ways it was asked
- “How would you implement the rate limiter?”
- “Imagine I ask you to design a rate limiter for an API. High level. How would you approach it?”
What is caching?
Asked in 3 of 587 interviews · last asked Oct 12 · Concept · Junior
Other ways it was asked
- “What is caching, and how does it work?”
- “What is caching, and when would you use it?”
What is the role of an API gateway?
Asked in 3 of 587 interviews · last asked Oct 1 · Concept · Mid-level
Other ways it was asked
- “What problem does an API gateway solve?”
- “Why do we need an API gateway?”
- “What is the purpose of a REST API gateway?”
What is eventual consistency?
Asked in 2 of 587 interviews · last asked Oct 15 · Concept · Mid-level
Other ways it was asked
- “What is eventual consistency, and where might users notice it?”

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What is a cascading failure?
Asked in 2 of 587 interviews · last asked Oct 6 · Concept · Mid-level
Other ways it was asked
- “How can you solve cascading failures?”
How does Kafka work?
Asked in 2 of 587 interviews · last asked Sep 4 · Concept · Mid-level
Other ways it was asked
- “How does Kafka work internally?”
How does the circuit breaker pattern work?
Asked in 2 of 587 interviews · last asked Oct 9 · Concept · Senior
Other ways it was asked
- “Can you explain the circuit breaker pattern?”
How do you design and implement batch processing?
Asked in 2 of 587 interviews · last asked Jun 10 · Concept · Mid-level
Other ways it was asked
- “What do you consider when designing and developing batch processing systems?”
How would you design a payment processing system?
Asked in 2 of 587 interviews · last asked Oct 9 · Design · Mid-level
Other ways it was asked
- “How would you design a payment system?”
- “How would you design the architecture of a payment gateway?”
- “How would you design authentication and authorization for a payment system?”
What are distributed transactions, and when would you use them?
Asked in 2 of 587 interviews · last asked Aug 19 · Concept · Mid-level
Other ways it was asked
- “What are some alternatives to distributed transactions?”

All System Design interview questions

Backend testing, DevOps and troubleshooting interview questions

Questions on running backend services cover testing, Git, CI/CD, containers, cloud deployment, monitoring, migrations, performance problems, and incident recovery. 517 questions.

What is the difference between Git merge and Git rebase?
Asked in 3 of 587 interviews · last asked Oct 7 · Concept · Junior
Other ways it was asked
- “What is the difference between Git merge and Git rebase, when would you use each, and what should you avoid?”
- “What is the difference between merging and rebasing in Git?”
What is CI/CD?
Asked in 3 of 587 interviews · last asked Mar 2 · Concept · Junior
Other ways it was asked
- “What is CI/CD, and how does it fit into deploying code to production?”
What is Docker, and what is it used for?
Asked in 3 of 587 interviews · last asked Aug 25 · Concept · Junior
Other ways it was asked
- “What is Docker?”
- “What are Docker and containers?”
What is Test-Driven Development, and when would you use it?
Asked in 3 of 587 interviews · last asked Jun 25 · Concept · Mid-level
Other ways it was asked
- “What is test-driven development (TDD), and how does it guide the development process?”
- “What is test-driven development, and how does its development cycle work?”
What is the difference between unit tests and integration tests?
Asked in 2 of 587 interviews · last asked Oct 7 · Concept · Junior
Other ways it was asked
- “What is the difference between a unit test and an integration test?”
What is the difference between a Dockerfile and Docker Compose?
Asked in 2 of 587 interviews · last asked Aug 25 · Concept · Junior
Other ways it was asked
- “What is the difference between Docker and Docker Compose?”
How would you investigate the cause of a slow application?
Asked in 2 of 587 interviews · last asked May 11 · Scenario · Mid-level
Other ways it was asked
- “When an application becomes slow, how would you identify the root cause?”
- “What are some common causes of a slow application?”
How would you troubleshoot and resolve an issue in production?
Asked in 2 of 587 interviews · last asked Oct 2 · Scenario · Senior
Other ways it was asked
- “How do you typically troubleshoot issues in production?”
If the database is performing well but an API remains slow, what application-side issues would you investigate?
Asked in 2 of 587 interviews · last asked Oct 5 · Scenario · Mid-level
Other ways it was asked
- “What common issues would you investigate when diagnosing poor API performance?”
Tell me about a testing framework you built for a system that was difficult to test. What made your approach more effective than standard unit tests and mocks?
Asked in 2 of 587 interviews · last asked Oct 17 · Behavioral · Senior
Other ways it was asked
- “Tell me about a testing framework you built for a difficult-to-test system. What made the system hard to test, and how did the framework validate correctness beyond basic unit tests?”
How would you investigate and improve code that works correctly but runs very slowly?
Asked in 2 of 587 interviews · last asked May 8 · Scenario · Senior
Other ways it was asked
- “Given a piece of code that is running slower than expected, how would you systematically analyze and optimize its performance?”
When conducting load tests with tools such as JMeter, what aspects would you evaluate?
Asked in 2 of 587 interviews · last asked May 18 · Tooling · Mid-level
Other ways it was asked
- “What metrics do you measure in a JMeter load test?”

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How do you diagnose performance problems?
Asked in 2 of 587 interviews · last asked Oct 22 · Scenario · Mid-level
Other ways it was asked
- “How do you diagnose application performance issues?”
What is AWS Lambda?
Asked in 2 of 587 interviews · last asked Oct 15 · Concept · Junior
Other ways it was asked
- “What have you used AWS Lambda for, and how does a Lambda function work?”
What is Docker Compose used for?
Asked in 2 of 587 interviews · last asked Jun 3 · Concept · Junior
Other ways it was asked
- “What is Docker Compose, and what is it used for?”
What types of software testing do you know?
Asked in 2 of 587 interviews · last asked Aug 25 · Concept · Junior
Other ways it was asked
- “What types of software testing do you use, and how do they differ?”
What are end-to-end tests?
Asked in 2 of 587 interviews · last asked Sep 21 · Concept · Junior
Other ways it was asked
- “How do you implement end-to-end tests in code?”
How would you implement an application using AWS services?
Asked in 2 of 587 interviews · last asked Oct 22 · Tooling · Mid-level
Other ways it was asked
- “Which AWS services have you used, and how have you implemented them?”
- “How have you used AWS services in backend projects?”
How do you handle challenges that arise during the deployment process?
Asked in 2 of 587 interviews · last asked Oct 13 · Scenario
Other ways it was asked
- “How will you resolve a problem if one arises during deployment?”
What's the difference between unit tests, integration tests, and end-to-end tests? Why would you use each?
Asked in 2 of 587 interviews · last asked Oct 22 · Concept · Mid-level
Other ways it was asked
- “Do you know the difference between unit, integration, and end-to-end test cases?”

Backend developer behavioral and project interview questions

Project and teamwork questions cover business needs, domain knowledge, delivery trade-offs, unfamiliar systems, and responsible AI tool use. 351 questions.

What do you look for when reviewing code?
Asked in 3 of 587 interviews · last asked May 26 · Concept · Mid-level
Other ways it was asked
- “What do you prioritize when reviewing code?”
How do you respond when a team member is falling behind schedule?
Asked in 3 of 587 interviews · last asked May 26 · Scenario · Mid-level
Other ways it was asked
- “How would you respond if a team member were falling behind schedule?”
- “How would you respond if a team member fell behind schedule?”
How would you respond if you were assigned work involving a technology you had not used before?
Asked in 3 of 587 interviews · last asked Oct 20 · Scenario
Other ways it was asked
- “How would you respond if you were assigned a technology you had never used before?”
- “How would you approach being assigned a technology you have never used before?”
- “How would you approach a task involving technology or code you had not worked with before?”
Tell me about a technically challenging project, the challenges you faced, and the decisions and trade-offs you made to resolve them.
Asked in 2 of 587 interviews · last asked Oct 9 · Behavioral · Mid-level
Other ways it was asked
- “Tell me about a technically challenging project, what you were trying to accomplish, and why.”
What are the fundamental differences between Waterfall and Agile?
Asked in 2 of 587 interviews · last asked May 18 · Concept · Junior
Other ways it was asked
- “How do you compare waterfall and agile development?”
- “How do waterfall and agile development differ?”
How do you ensure the quality of the code your team delivers?
Asked in 2 of 587 interviews · last asked Aug 19 · Behavioral · Senior
Other ways it was asked
- “How do you ensure code quality across your team?”
- “How do you ensure code quality?”
What is especially important when developing financial systems?
Asked in 2 of 587 interviews · last asked May 18 · Case · Senior
Other ways it was asked
- “What do you consider especially important when developing financial or banking systems?”
What activities are involved in requirements definition?
Asked in 2 of 587 interviews · last asked Jun 17 · Concept · Mid-level
Other ways it was asked
- “What activities do you typically handle during the requirements-definition phase?”
If you were assigned to a challenging project, how would you overcome the difficulties?
Asked in 2 of 587 interviews · last asked May 31 · Scenario
Other ways it was asked
- “How would you overcome challenges if assigned to a demanding project?”
How do you write effective prompts for AI systems?
Asked in 2 of 587 interviews · last asked Oct 16 · Concept · Mid-level
Other ways it was asked
- “How do you write prompts for AI tools to support software development?”
How would you clarify and organize a client's ambiguous requirements?
Asked once · last asked May 26 · Scenario · Mid-level
How would you address a mismatch between a client's expectations and the development team's understanding?
Asked once · last asked May 26 · Scenario · Mid-level

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Can you walk me through a complex system design you led and the key trade-offs you made?
Asked once · last asked Oct 20 · Design · Senior
How do you select technologies for a software project, and who should be involved in that decision?
Asked once · last asked May 14 · Scenario · Senior
Other ways it was asked
- “How do you evaluate and select technologies for a software project?”
How do you refine and prioritize backlog items with a product owner in an agile team, and coordinate backend work with frontend requirements?
Asked once · last asked May 21 · Scenario · Mid-level
What business processes are involved in Japan's statutory tax reporting?
Asked once · last asked May 25 · Case · Mid-level
How do you balance a client's priorities with the need to maintain technical quality?
Asked once · last asked May 26 · Scenario · Senior
What is spec-driven development, and when would you use it?
Asked once · last asked Jun 24 · Concept · Mid-level
What measurable change in API response time or system throughput resulted from a backend performance optimization you implemented?
Asked once · last asked Jul 16 · Behavioral · Mid-level
Other ways it was asked
- “Describe a backend performance optimization you carried out, how you diagnosed the issue, and what measurable improvement resulted.”

Live coding round next? Coding mode in the MeetAssist desktop app solves the problem on your screen, with your phone as the remote.

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 Backend Developer 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.

587 interviews, 305 candidates, Sep 2025 to Oct 2026. Updated every month with new interviews.

What should I focus on for a backend interview?

Which topics stand out for backend roles?

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Can I write the code, not just describe it?

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What Backend Developer interviews cover

The 20 most asked Backend Developer questions

How do processes and threads differ, and when would you choose one over the other?

What is dependency injection, and how does a dependency injection container work?

What is object-oriented programming, and what are its four main principles?

What are microservices, and when would you choose them over a monolith?

What is the difference between a list and a tuple in Python?

Write a function that solves the Two Sum problem. What is the time complexity of your solution?

How do you use AI tools in your day-to-day software development?

What are database indexes, and when would you use them?

What are method overloading and method overriding?

What is the difference between an interface and an abstract class?

What is polymorphism, and how does it work in Java?

How would you design payment idempotency, including key storage, validation, and responses, to prevent duplicate charges?

What are SQL joins, and how do inner, left outer, and full outer joins differ?

What are JWTs, what types are there, and how are they used for authentication?

Can you explain the SOLID principles?

What is the N+1 query problem, and how can you avoid it?

What is a HashMap, and how does it work internally in Java?

What is Python?

What is middleware, and how is it used in ASP.NET Core?

What is the difference between mutable and immutable objects?

Backend programming languages and frameworks interview questions

What is exception handling, and how are exceptions handled in Java?

What string hashing algorithm is used in Python?

What is the difference between StringBuffer and StringBuilder?

Can you explain Spring Boot?

What is the difference between Spring Framework and Spring Boot?

What are the different types of bean scopes in Spring?

What is the difference between `==` and `.equals()` in Java?

What is the difference between an array and an ArrayList?

What are decorators in Python?

What is Java?

What is garbage collection in Java?

What is the difference between IEnumerable and IQueryable in .NET?

How would you design an immutable class in Java?

Can you tell me the difference between a shallow copy and a deep copy in Python?

What is the difference between Java and JavaScript?

OOP and design patterns interview questions

What is inheritance, and what types of inheritance does Java support?

What are design patterns, and can you describe examples and when you would use them?

What is abstraction?

Can you describe the difference between transient, scoped, and singleton dependency injection lifetimes in .NET?

What is an interface in Java?

What is inversion of control?

Can you explain how Java handles multiple inheritance?

What are the types of dependency injection in Spring?

Can a static method be overridden?

What is the Singleton design pattern?

What are the main categories of software design patterns?

Tell me about a time you used a design pattern to solve a problem. What was the problem, and how did you solve it?

What is a captive dependency in .NET dependency injection, and why can it cause problems?

What makes code maintainable?

Backend coding and data structures interview questions

Write code to reverse a string.

What is a list?

What is an array?

What is a linked list, and how does it work?

Write code to find the largest number in an array.

Write code for the Two Sum problem.

Find the longest substring without repeated characters.

What is Dijkstra's algorithm, and when is it used?

What is the time complexity of merge sort and quicksort?

Given existing conference room bookings as an array of start- and end-time pairs and a new meeting request in the same format, write a function that determines whether the new meeting can be booked without overlapping any existing meeting.

Why would you choose breadth-first search (BFS) over depth-first search (DFS)?

How would you design and implement an LRU cache?

Write a function that prints “even” if a number x is even, or “odd” otherwise.

Design an LRU cache with get(key) and put(key, value) operations that run in O(1) time.

Write code to check whether parentheses are valid.

What should the maximum subarray sum be when all the numbers are negative?

In a HashMap, what exactly happens during a resize or rehash, how are buckets redistributed, and what determines which bucket an existing entry moves to?

How would you check whether a string is a palindrome?

Can you explain the difference between a stack and a queue, and give a real-world example of each?

Multithreading and async programming interview questions