Java Full Stack with React Interview Questions in Bangalore

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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.

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.

What does @SpringBootApplication do?

Spring BootEasy

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It is a convenience annotation combining three annotations. @SpringBootConfiguration marks the class as a configuration source. @EnableAutoConfiguration tells Boot to configure beans based on the classpath and properties, for example a DataSource when a JDBC driver is present. @ComponentScan scans the package of the main class and its sub-packages for components. That is why the main class should sit in the root package of your project.

Why do you want to become a Java full stack developer?

Core JavaEasy

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I enjoy building a feature end to end, from the database table to the screen the user clicks. Java and Spring Boot are used widely in Indian banking, product and services companies, so the skills stay in demand, and React lets me build modern interfaces. In my capstone project I built an inventory app with Spring Boot, MySQL and React, and owning the full flow taught me how each layer affects the user experience.

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

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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.

Tell me about a mistake you made with production data and how you handled it.

SQL and DatabasesMedium

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Situation: I ran an update script on production that missed a condition and changed status for more orders than intended. Task: correct it quickly and honestly. Action: I informed my lead immediately, stopped further jobs, used the audit table and a backup to restore the correct values and verified with the business team. Result: data was fixed within an hour. Since then, I always run scripts in a transaction with a row-count check first.

Compare HashMap, Hashtable and ConcurrentHashMap.

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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.

What are the correct and incorrect ways to use Optional?

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Use Optional as a return type when a value may be absent, and consume it with map, filter, orElse, orElseGet or orElseThrow. Avoid calling get() without checking, avoid Optional fields in entities or DTOs, avoid Optional parameters and never return null from a method that returns Optional. Prefer orElseGet over orElse when the default is expensive, because orElse always evaluates its argument.

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.

Compare promises and async/await.

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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.

How does JWT-based authentication work?

REST APIs, Security and MicroservicesMedium

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After a successful login, the server issues a JSON Web Token containing a header, a payload of claims such as user ID, roles and expiry, and a signature made with a secret or private key. The client sends it in the Authorization: Bearer header with each request. The server verifies the signature and expiry without a session lookup, which suits stateless APIs. Keep access tokens short-lived, do not store sensitive data in the payload because it is only encoded, and use refresh tokens for renewal.

What is the difference between DELETE, TRUNCATE and DROP?

SQL and DatabasesEasy

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DELETE removes selected rows using a WHERE clause, logs each row, fires triggers and can be rolled back. TRUNCATE removes all rows quickly by deallocating pages, resets identity counters in most databases and is treated as DDL, so rollback support depends on the database. DROP removes the whole table, including its structure, indexes and constraints. Use DELETE for business data and TRUNCATE for staging tables.

Write a reusable useFetch custom hook.

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export function useFetch(url) { const [state, setState] = useState({ data: null, loading: true, error: null }); useEffect(() => { let ignore = false; setState(s => ({ ...s, loading: true })); fetch(url) .then(r => r.ok ? r.json() : Promise.reject(new Error(r.statusText))) .then(data => { if (!ignore) setState({ data, loading: false, error: null }); }) .catch(error => { if (!ignore) setState({ data: null, loading: false, error }); }); return () => { ignore = true; }; }, [url]); return state; } // Usage: const { data, loading, error } = useFetch(`/api/orders/${id}`);

What is the difference between var, let and const?

JavaScript, HTML and CSSEasy

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var is function-scoped, hoisted and initialised as undefined, and can be redeclared, which causes bugs in loops and blocks. let and const are block-scoped and stay in the temporal dead zone until declared, so using them early throws an error. let allows reassignment and const does not, although a const object or array can still be mutated. Use const by default, let when reassignment is needed and avoid var.

Explain the different types of SQL joins.

SQL and DatabasesEasy

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INNER JOIN returns only rows that match in both tables. LEFT JOIN returns all rows from the left table plus matches from the right, with nulls where there is no match. RIGHT JOIN is the reverse. FULL OUTER JOIN returns all rows from both sides, which MySQL does not support directly. CROSS JOIN returns every combination. A self join joins a table to itself, such as employees and their managers.

What is the difference between controlled and uncontrolled components?

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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.

Add the JWT to every API call and handle 401 responses using an Axios interceptor.

React FrontendHard

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Tell me about a difficult bug you fixed in a Java project.

Core JavaMedium

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Situation: our order API sometimes returned another user's cart. Task: I had to find the root cause quickly. Action: I reproduced it with a load test, found a Spring singleton service storing the current user in an instance field, and moved that state into a method parameter. I added a concurrent test to prevent regressions. Result: the issue disappeared and I shared the learning about stateless beans with the team.

What is the difference between props and state?

React FrontendEasy

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Props are inputs passed from a parent component and are read-only inside the child. State is data owned by a component that can change over time, managed with useState or useReducer, and changing it triggers a re-render. For example, a ProductCard receives a product as props, while a cart counter keeps its quantity in state. When two components need the same state, lift it up to their common parent.

Write a unit test for a service method using JUnit 5 and Mockito.

Spring BootMedium

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@ExtendWith(MockitoExtension.class) class ProductServiceTest { @Mock ProductRepository repo; @InjectMocks ProductService service; @Test void throwsWhenProductMissing() { when(repo.findById(1L)).thenReturn(Optional.empty()); assertThrows(ResourceNotFoundException.class, () -> service.getProduct(1L)); verify(repo).findById(1L); } }

How do code splitting and lazy loading work in React?

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Code splitting breaks the JavaScript bundle into smaller chunks loaded on demand, which improves first load time. React.lazy with a dynamic import, such as const Reports = lazy(() => import('./Reports')), loads the component only when it renders, and Suspense shows a fallback while it loads. Splitting by route is the most common pattern. Bundlers like Vite create the chunks automatically from dynamic imports.

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.

Why should we hire you as a Java full stack developer?

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I can contribute on both sides of a feature: I have built REST APIs with Spring Boot, JPA and MySQL, and responsive React screens that consume them. I write tests, use Git properly and have deployed my projects with Docker. I also learn quickly; I picked up Spring Security and JWT for my capstone in two weeks. I am looking for a team where I can grow into an engineer who owns features from design to production.

Your React app's home page takes 6 seconds to load on mobile. How would you improve it?

JavaScript, HTML and CSSHard

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Measure with Lighthouse and the browser Performance tab, and check Core Web Vitals such as LCP and INP. Then reduce JavaScript with route-based code splitting and removing heavy libraries, compress and lazy-load images using modern formats like WebP or AVIF with proper sizes, enable gzip or Brotli and long-term caching through a CDN, and avoid render-blocking resources. Server-side rendering or pre-rendering, for example with Next.js, can improve first paint further.

Write global exception handling for a REST API in Spring Boot.

Spring BootMedium

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@RestControllerAdvice public class GlobalExceptionHandler { @ExceptionHandler(ResourceNotFoundException.class) public ProblemDetail handleNotFound(ResourceNotFoundException ex) { return ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, ex.getMessage()); } @ExceptionHandler(MethodArgumentNotValidException.class) public ProblemDetail handleValidation(MethodArgumentNotValidException ex) { ProblemDetail pd = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST); pd.setProperty("errors", ex.getBindingResult().getFieldErrors() .stream().map(e -> e.getField() + ": " + e.getDefaultMessage()).toList()); return pd; } }

What are intermediate and terminal operations in Java Streams?

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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.

Sort a list of products by category, then by price descending, then by name.

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products.sort( Comparator.comparing(Product::getCategory) .thenComparing(Product::getPrice, Comparator.reverseOrder()) .thenComparing(Product::getName)); // Null-safe variant for an optional field: Comparator<Product> byBrand = Comparator.comparing( Product::getBrand, Comparator.nullsLast(Comparator.naturalOrder()));

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.

How does the this keyword work, and how do arrow functions change it?

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In a regular function, this depends on how the function is called: the object before the dot in a method call, undefined in strict mode for a plain call, the new object with new, or a value set by call, apply or bind. Arrow functions do not have their own this; they use this from the surrounding scope. That makes arrows ideal for callbacks inside methods, but unsuitable as object methods that need their own this.

How do indexes improve performance, and when can they hurt?

SQL and DatabasesMedium

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An index, usually a B-tree, lets the database find rows without scanning the whole table, which speeds up WHERE, JOIN and ORDER BY on indexed columns. Indexes cost disk space and slow down INSERT, UPDATE and DELETE because each index must also be updated. They help little on low-selectivity columns like a boolean flag. Functions on the column, such as WHERE LOWER(email) = ?, or a leading wildcard in LIKE also prevent normal index use.

After calling setCount(count + 1), console.log(count) still shows the old value. Why?

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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.

Your team is starting a new React project. Would you use Create React App or something else?

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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.

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