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Java Full Stack with React Technical Round Questions & Answers

40 questions for the technical round stage — 3 easy, 29 medium, 8 hard.

About the technical round round

The technical round is the core of a Java Full Stack with React interview: concept questions with follow-ups, a live hands-on task and a deep dive into one of your projects. Interviewers check depth, not just definitions.

Tips for the technical round round

  • Say your approach out loud before you start writing.
  • After a correct answer, expect 'why?' and 'what if?' follow-ups; go one level deeper than the question.
  • Pick one project you can explain end to end, including what you would do differently.
  • If you get stuck, explain what you would try next instead of going silent.
Solved — technical roundStart practicing
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01

How do profiles and application.yml work in Spring Boot?

Spring BootMedium
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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.
02

What is CORS and how do you configure it for a React frontend?

REST APIs, Security and MicroservicesMedium
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CORS is a browser rule that blocks JavaScript from calling a different origin unless the server allows it with response headers like Access-Control-Allow-Origin. A React app on localhost:5173 calling an API on localhost:8080 is cross-origin. In Spring, define a CorsConfigurationSource bean with the allowed origins, methods and headers, and enable http.cors() in the security chain so preflight OPTIONS requests pass. Never use a wildcard origin together with credentials.
03

Count the frequency of each word in a sentence.

Collections and StreamsMedium
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String text = "java is fun and java is fast"; Map<String, Long> freq = Arrays.stream(text.toLowerCase().split("\\s+")) .collect(Collectors.groupingBy( Function.identity(), LinkedHashMap::new, Collectors.counting())); System.out.println(freq); // {java=2, is=2, fun=1, and=1, fast=1}
04

Return only a few columns using an interface-based projection.

JPA and HibernateMedium
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public interface ProductSummary { Long getId(); String getName(); BigDecimal getPrice(); } public interface ProductRepository extends JpaRepository<Product, Long> { List<ProductSummary> findByCategoryName(String category); @Query("select new com.shop.dto.PriceDto(p.name, p.price) from Product p where p.price < :max") List<PriceDto> cheaperThan(@Param("max") BigDecimal max); } // Projections select fewer columns and avoid loading full entities.
05

Inserting one lakh rows with saveAll takes many minutes. How do you speed it up?

JPA and HibernateHard
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Enable JDBC batching with hibernate.jdbc.batch_size of 50 or so, plus order_inserts and order_updates. IDENTITY ID generation disables insert batching in Hibernate, so use a SEQUENCE generator with an allocation size on PostgreSQL or Oracle. Flush and clear the persistence context every batch to keep memory low. For very large loads, use JdbcTemplate batchUpdate or the database's bulk loader like COPY or LOAD DATA, which is much faster than ORM.
06

Your Spring Boot service crashes with OutOfMemoryError: Java heap space in production. How do you investigate?

Core JavaHard
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First capture evidence: enable -XX:+HeapDumpOnOutOfMemoryError and check GC logs and container memory limits. Open the heap dump in Eclipse MAT or VisualVM and look at the dominator tree for the largest retained objects. Common causes are unbounded caches, loading full tables into lists, static collections that keep growing and large file reads. Fix the leak, add pagination or streaming, set a sensible -Xmx below the container limit and add memory alerts.
07

What is the difference between volatile and synchronized?

Core JavaHard
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volatile guarantees visibility: a write by one thread is immediately visible to others and reordering around it is restricted, but it does not make compound operations like count++ atomic. synchronized gives both mutual exclusion and visibility by acquiring a monitor lock, so only one thread runs the block at a time. Use volatile for simple flags such as a shutdown signal, synchronized or locks for check-then-act logic, and AtomicInteger or LongAdder for counters.
08

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

Core JavaEasy
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The == operator compares references for objects, meaning it checks whether two variables point to the same object in memory. equals() compares logical content when a class overrides it, as String, Integer and most collections do. For example, new String("a") == new String("a") is false but equals() returns true. For primitives, == compares values. Always use equals() for objects and Objects.equals(a, b) to stay null-safe.
09

Write a record and use pattern matching in a switch expression.

Core JavaMedium
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sealed interface Payment permits Card, Upi {} record Card(String number, double amount) implements Payment {} record Upi(String vpa, double amount) implements Payment {} static String describe(Payment p) { return switch (p) { case Card c when c.amount() > 50000 -> "High value card"; case Card c -> "Card ending " + c.number().substring(12); case Upi u -> "UPI to " + u.vpa(); }; } // Sealed types make the switch exhaustive without a default branch (Java 21).
10

What triggers a re-render in React, and how does reconciliation decide what to update?

React FrontendHard
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A component re-renders when its state changes, its parent re-renders, or a context it uses changes. Props changing is not itself the trigger; parent re-render is. During reconciliation React compares element types: a different type at the same position destroys and recreates the subtree, while the same type updates props. Keys identify list children. React.memo skips a child re-render when its props are shallowly equal.
11

What is the contract between equals() and hashCode()?

Collections and StreamsMedium
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If two objects are equal according to equals(), they must return the same hashCode. Unequal objects may share a hash code, but fewer collisions give better performance. If you override equals without hashCode, HashMap and HashSet will fail to find objects that are logically equal. Base both methods on the same immutable fields; records and IDE or Lombok generation create them correctly.
12

Find the top 3 highest-paid employees in each department.

SQL and DatabasesMedium
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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.
13

Your application fails at startup with a circular dependency error between two services. What do you do?

Spring BootMedium
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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.
14

Explain the CSS box model.

JavaScript, HTML and CSSEasy
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Every element is a box made of content, padding, border and margin, from inside to outside. By default, width applies only to the content, so padding and border add to the final size. Setting box-sizing: border-box makes width include padding and border, which makes layouts predictable, and most projects apply it to all elements. Vertical margins between block elements can collapse into one margin.
15

What are intermediate and terminal operations in Java Streams?

Collections and StreamsMedium
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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.
16

Explain the difference between map and flatMap in streams.

Collections and StreamsMedium
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map transforms each element into exactly one new element, so a Stream of orders mapped to customer names gives a Stream of String. flatMap transforms each element into a stream and flattens all of them into one stream. For example, orders.stream().flatMap(o -> o.getItems().stream()) gives every item from every order. Optional also has map and flatMap with the same idea.
17

The application fails with NoSuchBeanDefinitionException for a service. How do you debug it?

Spring BootEasy
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Check that the class has @Service or @Component, or is created by a @Bean method. Make sure it is inside the package of the main @SpringBootApplication class or a scanned sub-package. If there are several implementations, use @Qualifier or @Primary. Also check @Profile or @Conditional annotations that may disable it in the current environment. Running with --debug prints the condition report that shows why a bean was not created.
18

How do you version a REST API?

REST APIs, Security and MicroservicesMedium
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The most common approach is URI versioning, like /api/v1/orders and /api/v2/orders, because it is visible and easy to route and cache. Other options are a custom header or the Accept media type. Spring Framework 7 adds first-class API versioning support. Version only for breaking changes; adding optional fields is backward compatible. Keep old versions running for a published deprecation period, and document every version with OpenAPI.
19

A teammate replaced a simple for loop in a performance-critical path with a long stream pipeline. Is this a concern?

Collections and StreamsHard
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Streams are usually fine and readable, and the JIT optimises them well, but in very hot loops they can add allocation, boxing and lambda overhead. Check for boxing with Stream<Integer> instead of IntStream, and for repeated sorting or collecting. Measure with a JMH benchmark or profiler rather than guessing. Keep streams where readability matters and use a plain loop only where profiling proves a real bottleneck.
20

Write a production-friendly Dockerfile for a Spring Boot application.

Spring BootMedium
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FROM eclipse-temurin:21-jdk AS build WORKDIR /app COPY . . RUN ./mvnw -q -DskipTests package FROM eclipse-temurin:21-jre WORKDIR /app RUN useradd -r appuser USER appuser COPY --from=build /app/target/*.jar app.jar EXPOSE 8080 ENTRYPOINT ["java", "-XX:MaxRAMPercentage=75", "-jar", "app.jar"] # Multi-stage build keeps the image small; runs as non-root.
21

Compare promises and async/await.

JavaScript, HTML and CSSMedium
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A promise represents a value that will be available later, handled with then, catch and finally. async/await is syntax built on promises: an async function always returns a promise, and await pauses that function until the promise settles, so asynchronous code reads like synchronous code. Errors are handled with try/catch. Awaiting calls one by one is sequential, so use Promise.all for independent calls to run them in parallel.
22

What is event delegation and why is it useful?

JavaScript, HTML and CSSMedium
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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.
23

When should you use useMemo and useCallback?

React FrontendMedium
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useMemo caches the result of an expensive calculation until its dependencies change, such as filtering and sorting a big list. useCallback caches a function reference, which matters when passing callbacks to children wrapped in React.memo or used in effect dependencies. Do not wrap everything, because memoization itself has a cost. The React Compiler, released as stable in 2025, can add much of this memoization automatically in supported projects.
24

What is a custom hook and why would you write one?

React FrontendMedium
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A custom hook is a function whose name starts with use and which calls other hooks, letting you reuse stateful logic across components. Examples are useFetch for loading data, useDebounce for search input and useAuth for the current user. Each component calling the hook gets its own independent state. Custom hooks keep components focused on rendering and make logic easier to test, much like extracting a service in backend code.
25

Create a protected route in React Router that redirects unauthenticated users to login.

React FrontendHard
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function ProtectedRoute({ roles }) { const { user } = useAuth(); const location = useLocation(); if (!user) { return <Navigate to="/login" replace state={{ from: location }} />; } if (roles && !roles.includes(user.role)) { return <Navigate to="/unauthorized" replace />; } return <Outlet />; } // Routes <Route element={<ProtectedRoute roles={['ADMIN']} />}> <Route path="/admin" element={<AdminDashboard />} /> </Route> // The backend must still enforce authorization on every API.
26

What roles do an API gateway and service discovery play in microservices?

REST APIs, Security and MicroservicesMedium
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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.
27

Compare HashMap, Hashtable and ConcurrentHashMap.

Collections and StreamsMedium
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HashMap is not synchronized and allows one null key and null values. Hashtable is a legacy class where every method is synchronized, so it is slow and allows no nulls. ConcurrentHashMap is the modern thread-safe choice: reads are mostly lock-free, writes lock only a single bucket using CAS and synchronized blocks, and it offers atomic methods such as computeIfAbsent and merge. It also does not allow null keys or values.
28

Implement pagination and sorting for a product list API.

JPA and HibernateMedium
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@GetMapping("/api/products") public Page<ProductDto> list( @RequestParam(defaultValue = "0") int page, @RequestParam(defaultValue = "20") int size, @RequestParam(defaultValue = "price") String sort) { Pageable pageable = PageRequest.of(page, Math.min(size, 100), Sort.by(sort).ascending()); return repository.findByActiveTrue(pageable).map(ProductDto::from); } // Repository: Page<Product> findByActiveTrue(Pageable pageable);
29

What is a closure in JavaScript? Give a practical use.

JavaScript, HTML and CSSMedium
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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.
30

Database passwords and API keys are committed in application.properties. How do you fix this?

Spring BootMedium
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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.
31

Your public API is being hit by too many requests from a few clients. How do you add rate limiting?

REST APIs, Security and MicroservicesMedium
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Apply rate limiting at the edge first, in the API gateway, Nginx or the cloud load balancer, keyed by API key, user ID or IP. In Spring Cloud Gateway use the Redis rate limiter, or inside a service use Bucket4j with a token bucket backed by Redis so limits work across instances. Return 429 Too Many Requests with a Retry-After header. Different plans or endpoints like login can have stricter limits.
32

How does garbage collection work in modern Java?

Core JavaMedium
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The GC frees objects that are no longer reachable from GC roots such as thread stacks and static fields. Most objects die young, so the heap is generational: minor collections clean the young generation often and survivors are promoted. G1 is the default collector since Java 9 and targets predictable pause times by collecting regions. ZGC and Shenandoah offer sub-millisecond pauses for large heaps. Tuning usually starts with -Xmx and choosing the collector, not dozens of flags.
33

When would you use the Context API and when a state library like Redux Toolkit or Zustand?

React FrontendMedium
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Context is good for passing low-frequency global values such as theme, language or the logged-in user without prop drilling. Every consumer re-renders when the context value changes, so it is not ideal for fast-changing data. Redux Toolkit or Zustand suit larger shared client state with many updates, selectors and dev tools. Server data like API responses is often best handled by TanStack Query, which adds caching and refetching.
34

Write a debounce function in JavaScript.

JavaScript, HTML and CSSMedium
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function debounce(fn, delay = 300) { let timer; return function (...args) { clearTimeout(timer); timer = setTimeout(() => fn.apply(this, args), delay); }; } const search = debounce(term => { fetch(`/api/products?q=${encodeURIComponent(term)}`); }, 400); input.addEventListener('input', e => search(e.target.value)); // Only the last call within 400 ms actually runs.
35

Call three APIs in parallel and handle failure of any one gracefully.

JavaScript, HTML and CSSMedium
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async function loadDashboard() { const results = await Promise.allSettled([ fetch('/api/orders').then(r => r.json()), fetch('/api/stats').then(r => r.json()), fetch('/api/alerts').then(r => r.json()), ]); const [orders, stats, alerts] = results.map(r => r.status === 'fulfilled' ? r.value : null); return { orders, stats, alerts }; } // Promise.all rejects on the first failure; // allSettled lets the page show partial data.
36

Explain the JavaScript event loop, including microtasks and macrotasks.

JavaScript, HTML and CSSHard
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JavaScript runs on a single thread with a call stack. Asynchronous work is handled by the browser or Node, and callbacks wait in queues. After the stack empties, the event loop runs all microtasks first, such as promise callbacks and queueMicrotask, then one macrotask, such as setTimeout or an I/O callback, then renders if needed. So Promise.resolve().then(a) runs before setTimeout(b, 0). Long synchronous code blocks everything, including UI updates.
37

Write a query that shows daily sales with a running total for the month.

SQL and DatabasesMedium
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SELECT order_date, SUM(amount) AS daily_sales, SUM(SUM(amount)) OVER ( ORDER BY order_date ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW ) AS running_total FROM orders WHERE order_date >= DATE '2026-10-01' AND order_date < DATE '2026-11-01' GROUP BY order_date ORDER BY order_date;
38

In a search box, older API responses sometimes overwrite newer results. How do you fix this race condition?

React FrontendHard
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Each keystroke starts a request, and responses can arrive out of order. Debounce the input by about 300 milliseconds so fewer requests are sent. In the effect, cancel the previous request with AbortController in the cleanup function, or keep an ignore flag so stale responses are discarded. Libraries like TanStack Query handle this automatically by keying queries on the search term and ignoring outdated results.
39

Your code has frequent NullPointerExceptions from repository lookups. How would you redesign it?

Core JavaMedium
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Return Optional from lookup methods so callers must handle the missing case, for example findById(id).orElseThrow(). Validate inputs at the API boundary with Bean Validation, initialise collections to empty lists instead of null and use Objects.requireNonNull in constructors. Java 17 helpful NPE messages show which variable was null, which speeds debugging. Do not use Optional for fields or method parameters; it is meant for return values.
40

What is Spring Boot Actuator used for?

Spring BootMedium
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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.

Frequently asked questions

What happens in the Java Full Stack with React technical round round?

The technical round is the core of a Java Full Stack with React interview: concept questions with follow-ups, a live hands-on task and a deep dive into one of your projects. Interviewers check depth, not just definitions.

How do I clear the technical round round?

Say your approach out loud before you start writing. After a correct answer, expect 'why?' and 'what if?' follow-ups; go one level deeper than the question. Pick one project you can explain end to end, including what you would do differently.

Which round comes next?

A typical order is online assessment, telephonic screening, one or two technical rounds, a managerial round and HR. Use the other round pages to practise each stage.

Practise every interview round

Online Assessment questionsTelephonic Interview questionsManagerial Round questionsRapid-Fire questionsHR & behavioural questionsFull mock interview

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