Top 20 Java Interview Questions to Ask in 2025

Here’s a list of the Top 20 essential technical interview questions to ask when you hire Java programmers for senior experience levels.

Java Interview Questions for All Experience Levels

Let’s take a look at the curated list of questions, what answers to expect, and skills to evaluate.

1. How would you tune Hibernate’s session factory settings in a highly concurrent Java application?

This question helps you analyze if a candidate has the knowledge of Hibernate beyond simple ORM usage and can manage it under high concurrency. Strong responses should highlight session management, caching layers, and how pooling or batching methods can lower overhead. Besides this, you also need to evaluate how well they connect Hibernate configurations to broad system performance, which includes JVM behavior and database tuning. Furthermore, the candidate should be able to prioritize monitoring, profiling, and iterative optimization.

2. How can you design a low-latency, high-throughput Java application using Spring frameworks?

Here, you are evaluating the candidate’s ability to design systems that reduce latency without eliminating throughput, especially in large scale spring applications. Candidates might discuss reactive patterns, handling blocking calls properly, and implementing caching or queueing wherever necessary. The best answers highlight system-level thinking, understand trade-offs between concurrency models, measures to prevent bottlenecks, and methods to continuously measure performance. 

Thus, a strong candidate may display how they can design considering user experience and operational reliability in mind.

3. What strategies would you employ to enhance the security of a Java Spring Boot application communicating with external APIs?

This question evaluates how security is built into development. So, the ideal answers must address authorization, authentication and safe communication when interacting with external APIs. You should expect them to explain security best practices such as token-based security, secure credential storage, and protection of sensitive data. 

Assess how well they are able to prevent security threats such as data interception, replay attacks and overexposure of endpoints. This also highlights their proficiency on general coding best practices and Spring security. Therefore, candidates who demonstrate awareness of compliance, practical security controls are highly preferred, as those who can secure applications in real world environments.

4. How would you architect a Java-based microservices system on AWS to be scalable and maintainable?

This question tests cloud-native design skills. Strong responses highlight decomposition into microservices, non parallel communication wherever necessary, and the ability to handle service discovery and orchestration. Candidates should connect maintainability with scalability, highlighting that they are capable of structuring services with clear responsibilities and automated deployment pipelines.

You should also assess whether they understand AWS services that fit well in this ecosystem and how they balance costs, team efficiency and reliability. Hence, an ideal candidate is one who is able to display a practical approach to scalability that avoids complexity and ensures resilience and maintainability over time.

5. Illustrate how you would optimize query performance for a Java application using both SQL and NoSQL databases.

A good response should include practical database tuning skills. Candidates should be able to demonstrate the ability to analyze query performance, avoid redundancy and choose the right trade-offs between speed and consistency. Besides this you need to assess whether they can recognize common challenges and apply different strategies depending on database type. 

Furthermore, it also reveals if they can tune ORMs or bypass them. Thus, an ideal candidate is one who can balance theoretical knowledge with practical performance tuning that demonstrates strong problem-solving abilities.

6. Explain how you would manage transactions across distributed microservices in Java, especially when dealing with eventual consistency.

This question reveals whether the candidate has knowledge on distributed systems. A strong candidate should talk about concepts like eventual consistency, message-driven co-ordination or compensating actions. You should assess how well they are able to grasp trade-offs like when to apply patterns like saga or orchestration versus choreography. 

You also need to evaluate their ability to explain reliability measures, idempotency, and error handling which highlights their proficiency. The answers should be more than just naming tools; it’s about their reasoning on correctness in distributed environments. Therefore, an ideal candidate is someone who can demonstrate both technical expertise and practical decision-making skills.

7. What insights would you gain from A/B testing your Java backend, and how would that guide your development decisions on AWS?

This question evaluates how well a candidate uses experimentation to drive improvements. Strong answers should showcase not just the mechanics of splitting traffic but also how to define measurable outcomes like error rate, latenc,y and cost effectiveness. Additionally, you also need to see whether they can explain A/B results in a way that guides resource allocation, system design or scaling strategies. 

Along with this, you should look for a structured, data-driven approach rather than guesswork. Thus, candidates who can link A/B testing to continuous improvement, cost optimization and business value are preferable as they can perfectly align technical performance with big organization goals of AWS.

8. Can you present an example of a complex system on AWS where you successfully applied the dependency inversion principle from SOLID patterns in Java?

This question assesses how well a candidate is capable of applying design principles in practice rather than just a recall theory. A strong answer needs to focus on decoupling business logic from enabling testability, infrastructure, and ensuring adaptability for the future. Besides this, evaluate how well they explain their reasoning like the benefit of abstraction and how that decision improved long-term maintainability. 

Moreover, it also shows their skills to design for scale and resilience in AWS environments while keeping systems flexible to change. Hence, ideal candidates are those who reflects not only architectural awareness but also clarity in explaining principles like dependency inversion benefit real-world projects.

9. Explain the difference between public, private, and protected access modifiers.

Explanation of accessibility scopes across classes, inheritances and packages demonstrates their understanding of encapsulation and controlled visibility. Proficiency of practical use cases, such as API design or security, highlights depth. If they only memorize definitions, they may lack practical application knowledge.

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

Explanation inheritance limitations, default methods and when to apply contracts vs abstract, it highlights their skills and knowledge in design flexibility. An ideal answer includes connecting these differences into extensibility and maintainability to show their structured and practical thinking.  

11. What is an immutable object? How do you create an Immutable object in Java?

Candidates need to answer this question by mentioning private fields, no setters, and final keyword usage to show how they implement immutability. If they further explain by connecting immutability to concurrency safety, caching and predictability, it means they can implement the concept properly in performance focused applications. 

12. How do you handle exceptions in Java?

Ability to differentiate between unchecked and checked exceptions, explanation of meaningful exception hierarchy, demonstrates their good error handling ability. Mentioning custom exceptions shows mature coding practices and maintainability knowledge.

13. Explain the concept of inheritance in Java through examples

The question reveals the ability of candidates to demonstrate how inheritance enables code reuse, overriding and polymorphism with warning against deep hierarchies which highlights proficiency in design. Ideal answers must include practical application in real scenarios with inheritance trade-offs.

14. What are the main principles of OOP?

Candidates must explain abstraction, encapsulation, polymorphism and inheritance and connect them to modular design which showcases their core OOP knowledge. When they connect these principles to real world Java applications, they demonstrates their skills to design scalable applications.

15. Can you write a regular expression to check if String is a number?

The candidate is expected to mention a regex and also offers edge cases like negatives, decimals or whitespace; it highlights problem solving ability. Look for readability on regex patterns which suggests their practical knowledge.

16. Can you tell us best practices you use while using threads in Java?

If the candidate mentions shared mutual state, application concurrency utilities, prevention of deadlocks, effective synchronization and shutdown, it shows their practical knowledge. Awareness of balancing safety with performance demonstrates their experience with multi- threading complexities.

17. Is it possible for two unequal objects to have the same hashcode

Demonstration of hash collisions and how equals complements hashcode, reveals their skillset and knowledge on the collection framework. In addition, if they explain impact on Hashmap or set performance, it showcases in-depth understanding of hashing trade-offs in real world situations.

18. What is a compile time constant in Java? What is the risk of using it?

Strong answers must mention inline final static values and explain about the risks of recompilation issues, it shows awareness of API design challenges. Look if they connect constants to binary compatibility risks highlights their long-term future oriented approach.

19. Why is synchronization important? Explain with relevant examples.

Explanation of synchronization for thread safety along with examples such as counters or bank transactions displays their conceptual clarity. Mentioning risks such as contention or deadlocks or alternatives like atomics/locks demonstrates proficiency in concurrency.

20. Write a Java program to create and throw custom exceptions.

Demonstration of skills with writing a custom exception class and explains when to use it, shows that they know error handling is about clarity. Proficiency of domain specific exceptions highlights great designing abilities.

Final Thoughts

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