Java Full Stack with React Interview Questions in Thiruvananthapuram
36 practice questions with answers for Java Full Stack with React interviews in Thiruvananthapuram.
Java Full Stack with React jobs in Thiruvananthapuram
Thiruvananthapuram's Technopark is one of India's oldest IT parks, hosting IT-services, product and engineering-services companies. Data and testing teams here often work with global clients in finance, travel and healthcare.
Below are 36 questions that match the mix of service, product and domain employers hiring java full stack developers in Thiruvananthapuram.
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01
Database passwords and API keys are committed in application.properties. How do you fix this?
Remove the secrets from the repository, rotate the exposed credentials immediately, and clean the history if needed. Replace values with placeholders such as ${DB_PASSWORD} and supply them through environment variables, Kubernetes Secrets or a vault like HashiCorp Vault or a cloud secrets manager. Add a pre-commit secret scanner, keep local values in an ignored file, and give each environment its own credentials with least privilege.
02
Write a Flyway migration to add a column and index to an existing table.
-- src/main/resources/db/migration/V3__add_sku_to_products.sql
ALTER TABLE products ADD COLUMN sku VARCHAR(40);
UPDATE products SET sku = CONCAT('SKU-', id) WHERE sku IS NULL;
ALTER TABLE products ALTER COLUMN sku SET NOT NULL;
CREATE UNIQUE INDEX ux_products_sku ON products (sku);
# application.yml
# spring.jpa.hibernate.ddl-auto: validate
# Flyway runs pending V-scripts in order at startup.
03
After calling setCount(count + 1), console.log(count) still shows the old value. Why?
State updates are scheduled, not applied immediately. The count variable in the current render is a constant snapshot, so logging right after setState shows the old value; the new value appears in the next render. React also batches multiple updates. When the next state depends on the previous one, use the updater form setCount(c => c + 1), which works correctly even when called several times. Use useEffect to react to the updated value.
04
You need a thread-safe singleton for a configuration loader. How do you implement it?
The simplest safe options are an enum singleton or the holder idiom. With the holder idiom, a private static nested class holds the instance, and the JVM's class loading guarantees lazy, thread-safe initialisation without synchronization. Double-checked locking also works but needs a volatile field. In a Spring application you usually do not write singletons at all, because beans are singleton-scoped by default and the container manages them.
05
How do profiles and application.yml work in Spring Boot?
Common settings go in application.yml, and environment-specific values go in files like application-dev.yml or application-prod.yml. You activate a profile with spring.profiles.active=prod, an environment variable SPRING_PROFILES_ACTIVE or a command-line argument. Profile files override the default values. Beans can also be limited with @Profile("dev"). Environment variables and command-line arguments have higher priority than files, which is useful in Docker and Kubernetes.
Normalization organises tables to reduce redundancy and update anomalies. 1NF requires atomic values with no repeating groups. 2NF requires 1NF and that every non-key column depends on the whole primary key, not part of a composite key. 3NF requires 2NF and no transitive dependency, so non-key columns depend only on the key. For example, store customer city in the customers table, not in every order row. Reporting systems sometimes denormalize for speed.
10
What is the N+1 select problem and how do you solve it?
It happens when one query loads N parent rows and then each lazy association fires a separate query, giving N+1 queries. For example, listing 100 orders and calling getCustomer on each. Solve it with JOIN FETCH in JPQL, @EntityGraph on the repository method, batch fetching with hibernate.default_batch_fetch_size, or DTO projections that select only the needed columns. Enable SQL logging or a statistics tool in development to catch it early.
11
What roles do an API gateway and service discovery play in microservices?
An API gateway, such as Spring Cloud Gateway, is the single entry point for clients. It handles routing, authentication, rate limiting, CORS and request logging. Service discovery lets services find each other without hard-coded addresses: with Eureka or Consul, services register themselves, and clients look them up. On Kubernetes, built-in Services and DNS usually replace a separate discovery server, and an ingress or gateway handles external traffic.
12
What are intermediate and terminal operations in Java Streams?
Intermediate operations such as filter, map, sorted and distinct return a new stream and are lazy; nothing runs until a terminal operation is called. Terminal operations such as collect, forEach, count, reduce and findFirst trigger processing and produce a result. Laziness allows short-circuiting: with findFirst, elements are processed one by one only until a match is found. A stream can be consumed only once.
13
What is the difference between Comparable and Comparator?
Comparable defines a class's natural ordering through compareTo, implemented inside the class itself, such as String or Integer. Comparator is a separate object with compare, so you can define many orderings without changing the class. Since Java 8, Comparator.comparing, thenComparing and reversed make multi-field sorting concise. Use Comparable for one obvious order and Comparator for business-specific sorting like price then rating.
14
How does @Transactional work, and why does calling it from the same class not work?
Spring wraps the bean in a proxy. When an external caller invokes a @Transactional method, the proxy starts a transaction, calls the method and commits, or rolls back on a RuntimeException or Error. If a method in the same class calls another @Transactional method through this, the call skips the proxy, so the annotation is ignored. Fix it by moving the method to another bean or restructuring the boundary. The annotation also has no effect on private methods.
15
Write code that runs tasks in parallel using ExecutorService and collects results.
SELECT department_id, name, salary
FROM (
SELECT department_id, name, salary,
DENSE_RANK() OVER (
PARTITION BY department_id
ORDER BY salary DESC) AS rnk
FROM employees
) ranked
WHERE rnk <= 3
ORDER BY department_id, rnk;
-- DENSE_RANK keeps ties; use ROW_NUMBER for exactly 3 rows.
17
A user's JWT is stolen and you need to invalidate it before expiry. How would you design for this?
Pure JWTs cannot be revoked, so keep access tokens short-lived, around 10 to 15 minutes, and use refresh tokens stored server-side and rotated on each use. On logout or theft, revoke the refresh token. For immediate blocking, keep a denylist of token IDs or a per-user token version in Redis and check it in the filter. Send refresh tokens in HttpOnly, Secure, SameSite cookies to reduce theft through XSS.
18
What is the difference between lazy and eager fetching?
Lazy fetching loads an association only when it is first accessed, using a proxy. Eager fetching loads it immediately with the parent. By default, @OneToMany and @ManyToMany are lazy, while @ManyToOne and @OneToOne are eager. Best practice is to make all associations lazy and fetch what each use case needs with JOIN FETCH or entity graphs, because eager loading often pulls large object graphs and causes extra queries.
19
Mentoring: how would you help a junior developer who keeps pushing untested Spring code?
Situation: a junior on my team often broke the build with untested changes. Task: help him improve without demotivating him. Action: I paired with him for a few sessions to write JUnit and Mockito tests for his own features, shared a short checklist and set up a CI rule that blocks merges without passing tests. Result: within a month his pull requests passed review faster, and he later helped another new joiner.
Actuator adds production-ready endpoints for monitoring and management, such as /actuator/health, /actuator/info, /actuator/metrics and /actuator/prometheus. Kubernetes uses health liveness and readiness groups for probes. Metrics are collected through Micrometer and can be exported to Prometheus, Datadog or other systems. Only expose the endpoints you need with management.endpoints.web.exposure.include and secure them, because endpoints like env or heapdump can leak sensitive data.
21
Your React app gets 403 Forbidden on POST requests to Spring Boot, but GET works. What is the likely cause?
With Spring Security enabled, CSRF protection is on by default and blocks POST, PUT and DELETE that lack a CSRF token, while GET is allowed. For a stateless JWT API, disable CSRF in the SecurityFilterChain. If you use session cookies, keep CSRF and send the token with CookieCsrfTokenRepository. Also check CORS, because a rejected preflight OPTIONS request or a missing role in authorization rules can produce the same symptom.
22
What is a closure in JavaScript? Give a practical use.
A closure is a function that remembers variables from the scope where it was created, even after that outer function has returned. For example, function counter() { let n = 0; return () => ++n; } returns a function that keeps its own private n. Closures are used for data privacy, function factories, event handlers, debounce and memoization, and React hooks rely on them. Stale closures in effects are a common bug when dependencies are missing.
23
What is the difference between checked and unchecked exceptions?
Checked exceptions extend Exception (but not RuntimeException) and must be caught or declared with throws, for example IOException or SQLException. Unchecked exceptions extend RuntimeException, such as NullPointerException or IllegalArgumentException, and the compiler does not force handling. Use checked exceptions for recoverable conditions the caller should handle, and unchecked ones for programming errors. Spring prefers unchecked exceptions, for example DataAccessException, to keep service code clean.
24
Add the JWT to every API call and handle 401 responses using an Axios interceptor.
Create React App is deprecated and no longer recommended. For a single-page app backed by a Spring Boot API, Vite is the usual choice because of its fast dev server and optimised builds. If you need server-side rendering, SEO or file-based routing, a framework like Next.js or React Router in framework mode is recommended by the React team. Choose based on SEO needs, hosting and the team's experience.
The heap stores objects and is shared by all threads; it is managed by the garbage collector and split into young and old generations. Each thread has its own stack holding method frames, local variables and references. Metaspace, in native memory since Java 8, stores class metadata. There is also the program counter per thread and the code cache for JIT-compiled code. Stack overflow comes from deep recursion, while OutOfMemoryError usually means heap or metaspace is full.
27
What is the difference between controlled and uncontrolled components?
In a controlled component, form input values live in React state and change through onChange, so React is the single source of truth, which makes validation and conditional UI easy. In an uncontrolled component, the DOM keeps the value and you read it through a ref when needed, which needs less code and suits simple forms or file inputs. Libraries like React Hook Form use uncontrolled inputs for better performance in large forms.
28
How do you handle a disagreement with a UI designer about a screen?
Situation: a designer wanted a complex animated table that would be slow on low-end phones. Task: protect performance without dismissing the design. Action: I built a quick prototype, showed the lag on a budget Android device and proposed a lighter animation with the same feel. Result: the designer agreed, the page stayed fast, and we started reviewing performance impact together during design discussions.
29
A search query that used to take milliseconds now takes 10 seconds as data grew. How do you troubleshoot?
Get the exact SQL from logs or the slow query log and run EXPLAIN ANALYZE to see whether it does a full scan, a bad join order or a big sort. Check for missing or unused indexes, functions on indexed columns, implicit type conversions and outdated statistics. Add the right composite index, rewrite the query, select fewer columns and paginate. For text search, use a full-text index or a search engine instead of LIKE '%term%'.
30
Explain the architecture of your last full stack project end to end.
In my last project, a React single-page app called a Spring Boot REST API secured with JWT. The API followed controller, service and repository layers, used Spring Data JPA with MySQL and Flyway for migrations, and exposed Swagger documentation. I deployed it with Docker on a cloud VM behind Nginx. My contribution was the order module and payment status flow, and I also wrote the unit tests and CI pipeline.
31
You are designing a new feature. When would you choose a relational database and when a NoSQL database?
Choose a relational database like PostgreSQL or MySQL when data is structured, relationships matter and you need transactions and joins, such as orders, payments and inventory. Choose NoSQL when you need a flexible schema or huge horizontal scale for simple access patterns: MongoDB for documents, Redis for caching and sessions, Cassandra for high-write time series and Elasticsearch for search. Many systems use both, with SQL as the source of truth.
32
Your application fails at startup with a circular dependency error between two services. What do you do?
Spring Boot 2.6 and later forbid circular references by default. The real fix is design: two services depending on each other usually means a responsibility is in the wrong place. Extract the shared logic into a third bean, or use an application event so one service notifies the other without a direct reference. Using @Lazy or allowing circular references in properties hides the problem and should only be a temporary workaround.
Events bubble up from the target element to its ancestors. With event delegation you attach one listener to a parent, such as a ul, and use event.target.closest('li') to find which child was clicked. This saves memory compared with a listener on every item and automatically works for items added later. It is commonly used for dynamic tables and lists. React uses a similar idea by attaching listeners at the root.
SELECT e.name AS employee,
e.salary,
m.name AS manager,
m.salary AS manager_salary
FROM employees e
JOIN employees m ON e.manager_id = m.id
WHERE e.salary > m.salary;
-- A self join treats the same table as two roles.
35
In a bidirectional @OneToMany relationship, what does mappedBy mean?
mappedBy marks the inverse side and names the field on the other entity that owns the relationship. The owning side, usually the @ManyToOne with the foreign key column, is the one Hibernate uses to write the join column. For example, Order has @OneToMany(mappedBy = "order") and OrderItem has @ManyToOne @JoinColumn(name = "order_id"). Always update both sides in helper methods such as addItem to keep the object graph consistent.
36
Why is constructor injection preferred over field injection?
Constructor injection makes dependencies explicit and lets fields be final, so the object is always fully initialised and immutable. It allows plain unit tests without Spring or reflection, because you can pass mocks to the constructor. A long constructor also signals that a class has too many responsibilities. Field injection with @Autowired hides dependencies and needs a container or reflection in tests. With a single constructor, @Autowired is not even needed.
Few big employers listed in Thiruvananthapuram?
Many Thiruvananthapuram candidates also apply to teams in Bangalore and to remote or hybrid roles. Practise the Bangalore set as well.
Which companies hire Java Full Stack with React professionals in Thiruvananthapuram?
Companies with large offices in Thiruvananthapuram include many IT services and product firms. Use their company pages below to practise employer-specific sets.
Are Java Full Stack with React interviews in Thiruvananthapuram different from other cities?
The questions depend more on the company than the city. Thiruvananthapuram's mix of employers decides the style: Data and testing teams here often work with global clients in finance, travel and healthcare
How do I find Java Full Stack with React jobs in Thiruvananthapuram?
Shortlist companies with Thiruvananthapuram offices, keep your CV focused on Java Full Stack with React projects, apply on company career pages and job portals with the Thiruvananthapuram filter, and practise the questions on this page before each interview.