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Java Full Stack with React Rapid-Fire Questions & Answers

50 questions for the rapid-fire stage — 32 easy, 18 medium.

About the rapid-fire round

Rapid-fire questions are quick definitions and one-line answers that interviewers fire at the start or end of a round to check your Java Full Stack with React basics. Aim to answer each in under 20 seconds.

Tips for the rapid-fire round

  • Answer in one or two sentences, then stop.
  • If you don't know, say so quickly and move on; don't guess at length.
  • Revise these the morning of the interview.
  • Practise with a friend who reads them in random order.
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01

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.
02

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.
03

Explain final, finally and finalize.

Core JavaEasy
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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.
04

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.
05

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

JavaScript, HTML and CSSMedium
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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.
06

Compare String, StringBuilder and StringBuffer.

Core JavaEasy
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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.
07

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.
08

What are window functions and how are they different from GROUP BY?

SQL and DatabasesMedium
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Window functions compute a value over a set of related rows while keeping every row in the result, unlike GROUP BY which collapses rows into one per group. They use the OVER clause with optional PARTITION BY and ORDER BY. Examples are ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD and running SUM. For instance, you can show each employee with their department's average salary in the same row.
09

What bean scopes does Spring support?

Spring BootMedium
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singleton is the default: one instance per application context. prototype creates a new instance every time the bean is requested. Web applications also have request, session and application scopes, and websocket scope. Singleton beans must be stateless or thread-safe because many requests share them. If a singleton needs a prototype or request-scoped bean, inject an ObjectProvider or use a scoped proxy so each use gets the correct instance.
10

What is the N+1 select problem and how do you solve it?

JPA and HibernateMedium
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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 is the difference between lazy and eager fetching?

JPA and HibernateMedium
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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.
12

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.
13

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.
14

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.
15

When would you use Flexbox and when CSS Grid?

JavaScript, HTML and CSSEasy
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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.
16

What are ACID properties?

SQL and DatabasesEasy
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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.
17

What is the difference between checked and unchecked exceptions?

Core JavaEasy
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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.
18

Why are semantic HTML and accessibility important?

JavaScript, HTML and CSSEasy
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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.
19

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.
20

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.
21

Which HTTP methods are safe and which are idempotent?

REST APIs, Security and MicroservicesEasy
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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.
22

When would you use ArrayList over LinkedList?

Collections and StreamsEasy
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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.
23

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.
24

What is the difference between controlled and uncontrolled components?

React FrontendMedium
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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.
25

What is the difference between WHERE and HAVING?

SQL and DatabasesEasy
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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.
26

What is the difference between var, let and const?

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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.
27

How does HashMap work internally?

Collections and StreamsMedium
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HashMap stores entries in an array of buckets. The key's hashCode is spread and masked to find a bucket index. Collisions are kept in a linked list, and since Java 8 a bucket converts to a red-black tree after 8 entries when capacity is at least 64, giving O(log n) worst case. When size exceeds capacity times load factor 0.75, the table doubles and entries are redistributed. get uses hashCode to find the bucket and equals to find the key.
28

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.
29

What are the correct and incorrect ways to use Optional?

Collections and StreamsMedium
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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.
30

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

Spring BootEasy
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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.
31

What is the difference between List, Set and Map?

Collections and StreamsEasy
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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.
32

What is the difference between Comparable and Comparator?

Collections and StreamsEasy
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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.
33

What does the static keyword mean in Java?

Core JavaEasy
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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.
34

What are the key principles of a RESTful API?

REST APIs, Security and MicroservicesEasy
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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.
35

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

SQL and DatabasesEasy
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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.
36

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

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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.
37

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

Collections and StreamsEasy
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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.
38

What is hoisting in JavaScript?

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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.
39

What changed when moving from Spring Boot 2 to Spring Boot 3?

Spring BootMedium
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Spring Boot 3 requires Java 17 or later and is built on Spring Framework 6. The biggest migration task is the switch from javax.* to jakarta.* packages for persistence, validation and servlet APIs. Spring Security 6 removed WebSecurityConfigurerAdapter in favour of SecurityFilterChain beans. Boot 3 also supports GraalVM native images, Micrometer observation and tracing, and from 3.2 the RestClient and virtual thread support.
40

What is React and what is the virtual DOM?

React FrontendEasy
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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.
41

Why does React need a key prop when rendering lists?

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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.
42

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

JPA and HibernateEasy
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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.
43

How does the useEffect dependency array work?

React FrontendMedium
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useEffect runs after the component renders. With no dependency array it runs after every render, with an empty array it runs once after mount, and with values like [userId] it runs again whenever those values change. Return a cleanup function to cancel subscriptions, timers or requests before the next run or on unmount. Missing dependencies cause stale data, so follow the exhaustive-deps lint rule. Effects are for syncing with external systems, not for computing derived values.
44

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.
45

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

REST APIs, Security and MicroservicesEasy
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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.
46

How do derived query methods work in Spring Data JPA?

JPA and HibernateEasy
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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.
47

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.
48

Compare monolithic and microservices architecture.

REST APIs, Security and MicroservicesMedium
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A monolith is one deployable application. It is simpler to build, test and debug, and suits small teams and early products. Microservices split the system into independently deployable services owned by separate teams, such as orders, payments and inventory, each with its own database. They allow independent scaling and releases but add network latency, distributed data, monitoring and deployment complexity. Many teams start with a modular monolith and split only when needed.
49

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

JavaScript, HTML and CSSEasy
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== 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.
50

What is the difference between authentication and authorization?

REST APIs, Security and MicroservicesEasy
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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.

Frequently asked questions

What happens in the Java Full Stack with React rapid-fire round?

Rapid-fire questions are quick definitions and one-line answers that interviewers fire at the start or end of a round to check your Java Full Stack with React basics. Aim to answer each in under 20 seconds.

How do I clear the rapid-fire round?

Answer in one or two sentences, then stop. If you don't know, say so quickly and move on; don't guess at length. Revise these the morning of the interview.

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

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