Skip to content
Java Full Stack with React
All coursesPlacement Report
‹ Back to previous sectionCourse homeAll page URLs

Easy Java Full Stack with React Interview Questions

63 easy questions with answers. Definitions and fundamentals every candidate must answer quickly and correctly.

Topics in the easy set

Core Java: 12 JavaScript, HTML and CSS: 11 SQL and Databases: 8 Collections and Streams: 7 REST APIs, Security and Microservices: 7 React Frontend: 7 Spring Boot: 6 JPA and Hibernate: 5
Solved — easyStart practicing
3 QUESTIONS FREESign up to unlock the full question bank and track your progress.Unlock every question and answer, then continue practising company-wise and package-wise.
3 questions free Unlock the rest in seconds
01

What is the difference between JDK, JRE and JVM?

Core JavaEasy
Save to ▾
Show answer
The JVM runs Java bytecode and handles memory, garbage collection and JIT compilation. The JRE is the JVM plus core class libraries needed to run programs. The JDK is the JRE plus development tools such as javac, jar, jshell and jdb. Since Java 11 a separate JRE is no longer shipped by Oracle, so developers install a JDK such as Temurin or Oracle JDK 17 or 21 and build smaller runtimes with jlink if needed.
02

Why is String immutable in Java?

Core JavaEasy
Save to ▾
Show answer
Once created, a String object cannot change. This allows the String pool to safely share literals, makes strings thread-safe without locking, lets hashCode be cached for fast HashMap keys, and protects security-sensitive values like file paths, class names and URLs from being changed after validation. Any operation such as concat or replace returns a new String, which is why heavy modification should use StringBuilder.
03

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

Core JavaEasy
Save to ▾
Show answer
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.
04

Compare String, StringBuilder and StringBuffer.

Core JavaEasy
Save to ▾
Show answer
String is immutable, so every change creates a new object. StringBuilder is mutable and fast but not thread-safe, which makes it the default choice for building strings inside a method or loop. StringBuffer is mutable and synchronized, so it is thread-safe but slower, and it is rarely needed today because strings are seldom shared across threads while being built. The compiler already uses optimized concatenation for simple + expressions outside loops.
05

What is the difference between checked and unchecked exceptions?

Core JavaEasy
Save to ▾
Show answer
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.
06

Explain final, finally and finalize.

Core JavaEasy
Save to ▾
Show answer
final is a keyword: a final variable cannot be reassigned, a final method cannot be overridden and a final class cannot be extended. finally is a block after try or catch that always runs, used for cleanup, although try-with-resources is now preferred. finalize() was an Object method called before garbage collection; it is deprecated for removal because its timing is unpredictable. Use Cleaner or try-with-resources instead.
07

What does the static keyword mean in Java?

Core JavaEasy
Save to ▾
Show answer
static members belong to the class, not to an instance. A static variable has one copy shared by all objects, a static method can be called without creating an object and cannot use this, and a static block runs once when the class loads. Static nested classes do not hold a reference to the outer object. Common examples are utility methods like Math.max and constants declared as public static final.
08

Show how to read a file safely using try-with-resources.

Core JavaEasy
Save to ▾
Show answer
Path path = Path.of("data.txt"); try (BufferedReader reader = Files.newBufferedReader(path)) { String line; while ((line = reader.readLine()) != null) { System.out.println(line); } } catch (IOException e) { log.error("Could not read file", e); } // The reader is closed automatically, even if an exception occurs, // because BufferedReader implements AutoCloseable.
09

Create a custom unchecked exception and throw it from a service method.

Core JavaEasy
Save to ▾
Show answer
public class ResourceNotFoundException extends RuntimeException { public ResourceNotFoundException(String message) { super(message); } } public Product getProduct(Long id) { return repository.findById(id) .orElseThrow(() -> new ResourceNotFoundException("Product not found: " + id)); } // A @RestControllerAdvice can map this exception to HTTP 404.
10

Write a Java method to check whether a string is a palindrome, ignoring case and spaces.

Core JavaEasy
Save to ▾
Show answer
static boolean isPalindrome(String s) { String clean = s.replaceAll("\\s+", "").toLowerCase(); int i = 0, j = clean.length() - 1; while (i < j) { if (clean.charAt(i++) != clean.charAt(j--)) { return false; } } return true; } // Two pointers give O(n) time and O(n) space for the cleaned copy. // isPalindrome("Nurses Run") returns true.
11

A report job builds a large string with + inside a loop and is very slow. What do you change?

Core JavaEasy
Save to ▾
Show answer
Each + inside a loop creates a new String and copies the old content, giving roughly O(n squared) work. Replace it with one StringBuilder created before the loop, optionally with an initial capacity, and call append inside the loop. For joining collections, use String.join or Collectors.joining. For very large output, write directly to a BufferedWriter or stream instead of holding everything in memory.
12

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

Core JavaEasy
Save to ▾
Show answer
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.
13

What is the difference between List, Set and Map?

Collections and StreamsEasy
Save to ▾
Show answer
A List is an ordered collection that allows duplicates and index access, such as ArrayList. A Set holds unique elements; HashSet has no order, LinkedHashSet keeps insertion order and TreeSet keeps elements sorted. A Map stores key-value pairs with unique keys, such as HashMap or TreeMap, and is not part of the Collection interface. Choose based on whether you need duplicates, ordering or lookup by key.
14

When would you use ArrayList over LinkedList?

Collections and StreamsEasy
Save to ▾
Show answer
ArrayList is backed by a resizable array, giving O(1) random access and good cache locality, and appending is amortised O(1). LinkedList is a doubly linked list with O(1) insertion at known nodes but O(n) access by index and more memory per element. In practice ArrayList is faster for almost all workloads. For queue or stack behaviour, ArrayDeque is usually better than LinkedList.
15

What is the difference between Comparable and Comparator?

Collections and StreamsEasy
Save to ▾
Show answer
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.
16

What is a functional interface and how do lambdas relate to it?

Collections and StreamsEasy
Save to ▾
Show answer
A functional interface has exactly one abstract method, such as Runnable, Comparator, Function, Predicate, Supplier or Consumer, and can be marked with @FunctionalInterface. A lambda expression is a short implementation of that single method, for example Predicate<String> empty = s -> s.isBlank(). Method references like String::toUpperCase are an even shorter form. Lambdas power the Streams API and callback-style code.
17

Find duplicate elements in a list of integers.

Collections and StreamsEasy
Save to ▾
Show answer
List<Integer> nums = List.of(4, 2, 7, 2, 4, 9); Set<Integer> seen = new HashSet<>(); Set<Integer> duplicates = nums.stream() .filter(n -> !seen.add(n)) .collect(Collectors.toSet()); System.out.println(duplicates); // [2, 4] // Stateless alternative using counts: // nums.stream().collect(groupingBy(identity(), counting())) // .entrySet().stream().filter(e -> e.getValue() > 1)
18

You need to store unique usernames and always display them in alphabetical order. Which collection do you pick?

Collections and StreamsEasy
Save to ▾
Show answer
A TreeSet fits because it rejects duplicates and keeps elements sorted using natural order or a Comparator, with O(log n) add and lookup. Use String.CASE_INSENSITIVE_ORDER as the comparator if Rahul and rahul should be treated as the same. If you only need sorting once at display time, a HashSet followed by sorting may be faster. For concurrent access, ConcurrentSkipListSet is the thread-safe sorted alternative.
19

How do you keep yourself updated with new Java versions and features?

Collections and StreamsEasy
Save to ▾
Show answer
I follow the six-monthly Java release notes and JEP summaries, and I try new features in small jshell experiments or side projects. For example, when Java 21 became LTS I rewrote a small file-processing tool using virtual threads and pattern matching to understand the benefits. I also read Spring release blogs and watch conference talks, and I share short notes with my team so we upgrade with confidence.
20

What is Spring Boot and how is it different from the Spring Framework?

Spring BootEasy
Save to ▾
Show answer
Spring Framework provides dependency injection, MVC, data access and transactions, but traditionally needed a lot of manual configuration and an external server. Spring Boot sits on top of Spring and adds auto-configuration, starter dependencies, an embedded Tomcat, Jetty or Undertow server, externalised configuration and production features like Actuator. You can run an application as a single executable jar with java -jar, which makes it ideal for microservices and containers.
21

Explain Inversion of Control and Dependency Injection.

Spring BootEasy
Save to ▾
Show answer
Inversion of Control means the framework, not your code, creates and wires objects. Dependency Injection is how Spring does it: a class declares what it needs, usually through its constructor, and the container supplies the beans. For example, OrderService receives an OrderRepository instead of creating one with new. This reduces coupling, makes it easy to swap implementations and lets you pass mocks in unit tests.
22

What does @SpringBootApplication do?

Spring BootEasy
Save to ▾
Show answer
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.
23

Create a scheduled job that runs every night at 2 AM in Spring Boot.

Spring BootEasy
Save to ▾
Show answer
@SpringBootApplication @EnableScheduling public class App { } @Component public class CleanupJob { private final SessionRepository repo; public CleanupJob(SessionRepository repo) { this.repo = repo; } @Scheduled(cron = "0 0 2 * * *", zone = "Asia/Kolkata") public void removeExpiredSessions() { repo.deleteByExpiresAtBefore(Instant.now()); } } // With several instances, use ShedLock so only one runs the job.
24

You need H2 for local development, MySQL for testing and PostgreSQL in production. How do you configure this?

Spring BootEasy
Save to ▾
Show answer
Create application-dev.yml, application-test.yml and application-prod.yml, each with its own spring.datasource URL, username and driver. Keep shared settings in application.yml. Activate the right one with SPRING_PROFILES_ACTIVE in each environment. Read production credentials from environment variables, for example url: ${DB_URL}, instead of hard-coding them. For realistic integration tests, Testcontainers can start the same database engine used in production.
25

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

Spring BootEasy
Save to ▾
Show answer
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.
26

What is the difference between JPA, Hibernate and Spring Data JPA?

JPA and HibernateEasy
Save to ▾
Show answer
JPA, now Jakarta Persistence, is a specification that defines annotations and the EntityManager API for object-relational mapping. Hibernate is the most widely used implementation of that specification and adds extra features. Spring Data JPA is a layer above JPA that generates repository implementations from interfaces, so methods like findByEmail are created automatically. In a Spring Boot app you write against Spring Data, which uses JPA, which is implemented by Hibernate.
27

How do derived query methods work in Spring Data JPA?

JPA and HibernateEasy
Save to ▾
Show answer
Spring Data parses the method name in a repository interface and builds the query. For example, findByEmailAndActiveTrue(String email) or findTop5ByCategoryOrderByPriceDesc(String category). Supported keywords include And, Or, Between, LessThan, Like, In, IsNull and OrderBy. Return types can be Optional, List, Page or Stream. For complex logic, use @Query with JPQL or native SQL, because very long method names become hard to read.
28

What are the options for spring.jpa.hibernate.ddl-auto and which should be used in production?

JPA and HibernateEasy
Save to ▾
Show answer
none does nothing, validate checks that the schema matches entities, update alters tables to add missing columns, create drops and recreates the schema at startup and create-drop also drops it at shutdown. update and create are fine for learning but risky in production because they cannot handle renames or data migrations safely. In production use validate or none, and manage schema changes with Flyway or Liquibase scripts.
29

Write a JPA entity for a Product with an auto-generated ID and basic constraints.

JPA and HibernateEasy
Save to ▾
Show answer
@Entity @Table(name = "products") public class Product { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; @Column(nullable = false, length = 120) private String name; @Column(nullable = false, precision = 10, scale = 2) private BigDecimal price; @ManyToOne(fetch = FetchType.LAZY) @JoinColumn(name = "category_id") private Category category; protected Product() {} // required by JPA }
30

How do you learn a new framework or library quickly when a project needs it?

JPA and HibernateEasy
Save to ▾
Show answer
I start with the official getting-started guide and build a tiny working example, then read how it is used in our codebase. For example, when our team adopted Flyway, I created a sample project, tried failure cases like a bad migration and wrote a one-page guide for the team. Within a week I was handling migrations in the main project. I also note questions and clarify them with seniors early instead of guessing.
31

What are the key principles of a RESTful API?

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
REST treats everything as a resource identified by a URL, such as /api/orders/42, and uses standard HTTP methods to act on it. Requests are stateless, so each one carries all the information needed, including the auth token. Responses use proper status codes and representations, usually JSON, and can be cacheable. Good REST design uses nouns in URLs, plural resource names, nested paths for relations and query parameters for filtering and pagination.
32

Which HTTP methods are safe and which are idempotent?

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
Safe methods do not change server state: GET, HEAD and OPTIONS. Idempotent methods give the same result if repeated: GET, PUT, DELETE, HEAD and OPTIONS. POST is neither, so retrying a POST can create duplicate orders, which is why payment APIs use an idempotency key header. PATCH is not guaranteed idempotent. Knowing this matters for retries in clients, gateways and message consumers.
33

Explain the common HTTP status codes a REST API should return.

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
200 OK for successful reads and updates, 201 Created with a Location header after creating a resource, and 204 No Content for successful deletes. 400 Bad Request for validation errors, 401 Unauthorized when authentication is missing or invalid, 403 Forbidden when the user is authenticated but not allowed, 404 Not Found, 409 Conflict for version or duplicate conflicts, and 429 Too Many Requests for rate limits. 500 means an unexpected server error and 503 means temporarily unavailable.
34

What is the difference between authentication and authorization?

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
Authentication verifies who the user is, for example by checking a password, OTP or token. Authorization decides what that authenticated user is allowed to do, for example only admins can delete products. In Spring Security, authentication produces an Authentication object with granted authorities, and authorization uses those authorities in URL rules or annotations like @PreAuthorize("hasRole('ADMIN')"). A failed authentication gives 401 and a failed authorization gives 403.
35

Write a basic CRUD REST controller in Spring Boot.

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
@RestController @RequestMapping("/api/products") public class ProductController { private final ProductService service; public ProductController(ProductService service) { this.service = service; } @GetMapping("/{id}") public ProductDto get(@PathVariable Long id) { return service.get(id); } @PostMapping public ResponseEntity<ProductDto> create(@Valid @RequestBody ProductRequest req) { ProductDto saved = service.create(req); return ResponseEntity.created(URI.create("/api/products/" + saved.id())).body(saved); } @DeleteMapping("/{id}") @ResponseStatus(HttpStatus.NO_CONTENT) public void delete(@PathVariable Long id) { service.delete(id); } }
36

How do you store user passwords securely in Spring Boot? Show the code.

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
@Bean PasswordEncoder passwordEncoder() { return new BCryptPasswordEncoder(12); } public void register(RegisterRequest req) { User user = new User(req.email(), encoder.encode(req.password())); userRepository.save(user); } public boolean checkLogin(String raw, User user) { return encoder.matches(raw, user.getPasswordHash()); } // BCrypt adds a random salt and is deliberately slow. // Never store plain text or use MD5/SHA-1 for passwords.
37

How do you work with frontend developers and testers on API changes?

REST APIs, Security and MicroservicesEasy
Save to ▾
Show answer
I agree on the API contract first using an OpenAPI spec or Swagger UI, so frontend and QA can start in parallel with mock data. I share example requests and error responses, and I tell the team early if a field must change. For example, in my last sprint I gave QA a Postman collection with test cases for every endpoint. This reduced back-and-forth and our feature passed testing on the first cycle.
38

What is the difference between a primary key and a unique key?

SQL and DatabasesEasy
Save to ▾
Show answer
A primary key uniquely identifies each row, cannot be null, and a table has only one, although it may span several columns. A unique key also enforces uniqueness but a table can have many, and most databases allow null values in it. Both create an index automatically. For example, id is the primary key of a users table while email and mobile have unique constraints.
39

Explain the different types of SQL joins.

SQL and DatabasesEasy
Save to ▾
Show answer
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.
40

What is the difference between WHERE and HAVING?

SQL and DatabasesEasy
Save to ▾
Show answer
WHERE filters individual rows before grouping and cannot use aggregate functions. HAVING filters groups after GROUP BY and can use aggregates like COUNT or SUM. For example, SELECT dept, COUNT(*) FROM employees WHERE active = true GROUP BY dept HAVING COUNT(*) > 10. Put conditions in WHERE whenever possible because filtering earlier reduces the rows the database must group.
41

What are ACID properties?

SQL and DatabasesEasy
Save to ▾
Show answer
Atomicity means a transaction fully succeeds or fully rolls back. Consistency means it moves the database from one valid state to another, respecting constraints. Isolation means concurrent transactions do not see each other's partial work. Durability means committed data survives crashes, thanks to write-ahead logs. A bank transfer that debits one account and credits another is the classic example where all four properties matter.
42

What is the difference between DELETE, TRUNCATE and DROP?

SQL and DatabasesEasy
Save to ▾
Show answer
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.
43

Write a query to find the second highest salary from an employees table.

SQL and DatabasesEasy
Save to ▾
Show answer
-- Using a subquery SELECT MAX(salary) AS second_highest FROM employees WHERE salary < (SELECT MAX(salary) FROM employees); -- Using DISTINCT with OFFSET SELECT DISTINCT salary FROM employees ORDER BY salary DESC LIMIT 1 OFFSET 1; -- The first query returns NULL when there is no second salary.
44

Write a query to find duplicate email addresses in a users table and delete the extra rows.

SQL and DatabasesEasy
Save to ▾
Show answer
-- Find duplicates SELECT email, COUNT(*) AS cnt FROM users GROUP BY email HAVING COUNT(*) > 1; -- Keep the lowest id, delete the rest (MySQL / PostgreSQL) DELETE FROM users u USING users k WHERE u.email = k.email AND u.id > k.id; -- PostgreSQL syntax -- Then add: ALTER TABLE users ADD CONSTRAINT ux_email UNIQUE (email);
45

A security review found a login query built by string concatenation. What is the risk and the fix?

SQL and DatabasesEasy
Save to ▾
Show answer
Concatenating user input into SQL allows SQL injection, where input like ' OR '1'='1 changes the query and can bypass login or dump data. Use parameterised queries: PreparedStatement with ? placeholders, Spring Data method queries, or @Query with named parameters, which send values separately from the SQL. Also validate input, use a database user with least privilege and never show raw SQL errors to users.
46

What is React and what is the virtual DOM?

React FrontendEasy
Save to ▾
Show answer
React is a JavaScript library for building user interfaces from reusable components. Components describe what the UI should look like for a given state, and React updates the browser DOM when the state changes. The virtual DOM is a lightweight in-memory tree of the UI. On each update, React compares the new tree with the previous one and applies only the minimum real DOM changes, which keeps updates efficient and the code declarative.
47

What is the difference between props and state?

React FrontendEasy
Save to ▾
Show answer
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.
48

Why does React need a key prop when rendering lists?

React FrontendEasy
Save to ▾
Show answer
Keys help React match list items between renders, so it knows which items were added, removed or moved. Use a stable unique ID from the data, such as product.id. Using the array index as key causes bugs when items are reordered or deleted, because state and input values can attach to the wrong row. Keys only need to be unique among siblings, and they are not passed to the component as a prop.
49

How do you take feedback from a senior on your React code?

React FrontendEasy
Save to ▾
Show answer
Situation: in my first month, a senior reviewer left many comments about large components and duplicated fetch logic. Task: improve the code and learn from it. Action: I thanked him, asked for a short call to understand the reasoning, split the component, and wrote a custom hook for data fetching. Result: the pull request was approved, and the hook was reused in three other screens. I now self-review using his checklist.
50

Implement a cart counter using useReducer.

React FrontendEasy
Save to ▾
Show answer
function cartReducer(state, action) { switch (action.type) { case 'add': return { count: state.count + 1 }; case 'remove': return { count: Math.max(0, state.count - 1) }; case 'reset': return { count: 0 }; default: throw new Error('Unknown action ' + action.type); } } function Cart() { const [state, dispatch] = useReducer(cartReducer, { count: 0 }); return ( <div> <span>Items: {state.count}</span> <button onClick={() => dispatch({ type: 'add' })}>+</button> <button onClick={() => dispatch({ type: 'remove' })}>-</button> </div> ); }
51

You are passing the same user prop through five levels of components. What would you do?

React FrontendEasy
Save to ▾
Show answer
This is prop drilling, which makes intermediate components depend on data they do not use. For global, rarely changing data like the logged-in user or theme, create a context with a provider at the top and read it with useContext through a custom hook such as useAuth. Alternatively, use component composition, passing ready-made elements as children. For complex shared state, a store like Zustand or Redux Toolkit is suitable.
52

How do you handle a disagreement with a UI designer about a screen?

React FrontendEasy
Save to ▾
Show answer
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.
53

What is the difference between var, let and const?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
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.
54

What is the difference between == and === in JavaScript?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
== compares after type coercion, so 0 == '0' and null == undefined are true, which leads to surprising results. === compares both type and value without coercion, so 0 === '0' is false. Always use === and !== in application code. The only common exception is value == null, which checks for both null and undefined in one expression.
55

What is hoisting in JavaScript?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
During compilation, declarations are moved to the top of their scope. Function declarations are hoisted fully, so they can be called before they appear. var declarations are hoisted and initialised to undefined. let, const and class declarations are hoisted but stay uninitialised in the temporal dead zone, so accessing them early throws a ReferenceError. Function expressions assigned to variables follow the variable's rules.
56

Explain the CSS box model.

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
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.
57

When would you use Flexbox and when CSS Grid?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
Flexbox is one-dimensional: it arranges items in a row or a column and is great for navbars, button groups, centring and card rows that wrap. Grid is two-dimensional: it controls rows and columns together, ideal for page layouts, dashboards and galleries with grid-template-areas. They work well together, for example a Grid page layout with Flexbox inside each card. Both reduce the need for floats and positioning hacks.
58

Why are semantic HTML and accessibility important?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
Semantic elements like header, nav, main, article, button and label describe meaning, so screen readers, search engines and browsers understand the page. Use a real button instead of a clickable div so keyboard and focus work automatically. Add alt text to images, link labels to inputs, keep heading order logical and ensure enough colour contrast. Accessibility follows WCAG guidelines and improves SEO and usability for everyone.
59

Use map, filter and reduce to get the total value of in-stock products.

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
const products = [ { name: 'Pen', price: 20, qty: 10, inStock: true }, { name: 'Bag', price: 900, qty: 0, inStock: false }, { name: 'Book', price: 250, qty: 4, inStock: true }, ]; const total = products .filter(p => p.inStock) .map(p => p.price * p.qty) .reduce((sum, value) => sum + value, 0); console.log(total); // 1200
60

Centre a div horizontally and vertically using CSS.

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
.container { display: flex; justify-content: center; /* horizontal */ align-items: center; /* vertical */ min-height: 100vh; } /* Grid alternative */ .container { display: grid; place-items: center; min-height: 100vh; }
61

Write CSS for a product grid that shows 1 column on mobile, 2 on tablet and 4 on desktop.

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
.grid { display: grid; grid-template-columns: 1fr; gap: 16px; padding: 0 16px; } @media (min-width: 640px) { .grid { grid-template-columns: repeat(2, 1fr); } } @media (min-width: 1024px) { .grid { grid-template-columns: repeat(4, 1fr); } } /* Mobile-first: base styles for small screens, then widen. */
62

The layout looks fine on desktop but breaks on mobile with horizontal scrolling. How do you debug it?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
Check that the page has the viewport meta tag. Use browser device mode to find the element wider than the screen, often fixed widths in pixels, large images, long words or tables. Replace fixed widths with max-width: 100 percent, flexible units and flex-wrap, add overflow-wrap for long text and wrap wide tables in a scrollable container. Build mobile-first with media queries that add complexity for larger screens.
63

How do you handle a Git merge conflict when working in a team?

JavaScript, HTML and CSSEasy
Save to ▾
Show answer
Situation: two of us edited the same service class in parallel branches. Task: merge without losing either change. Action: I pulled the latest main, rebased my branch, opened the conflicted files and resolved them together with my teammate on a quick call, then ran all tests before pushing. Result: the merge was clean. To reduce conflicts, we now keep branches short-lived and pull from main daily.

Frequently asked questions

Who should practise easy Java Full Stack with React questions?

Freshers and career switchers should start here, and experienced candidates should skim them the day before an interview.

How should I practise these?

Answer each question aloud before opening the answer, mark it as solved only when your answer matches the key idea, and save misses to Last-Minute Revision.

Other difficulty levels

Medium questionsHard questions

More ways to practise

Company-wise Java Full Stack with React questionsExperience-wise Java Full Stack with React questionsRole-wise Java Full Stack with React questionsCity-wise Java Full Stack with React questionsMost asked Java Full Stack with React questionsScenario-based questionsPractical / hands-on questionsHR & behavioural questionsJava Full Stack with React mock interviewOnline assessment questionsTechnical round questionsManagerial round questionsJava Full Stack with React topic sheets