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Java Full Stack with React Telephonic Interview Questions & Answers

30 questions for the telephonic interview stage — 19 easy, 11 medium.

About the telephonic interview round

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Java Full Stack with React concepts clearly without a screen to point at.

Tips for the telephonic interview round

  • Keep your CV in front of you and be ready to explain every line on it.
  • Answer in short, clear sentences; there is no whiteboard, so structure matters.
  • Take the call somewhere quiet and confirm the time in advance.
  • Prepare one question to ask the interviewer at the end.
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01

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.
02

What is the difference between persist and merge, and between save and saveAndFlush?

JPA and HibernateMedium
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persist makes a new entity managed and assigns its ID; it fails for detached entities. merge copies the state of a detached entity onto a managed copy and returns that copy. Spring Data save calls persist for new entities and merge otherwise. saveAndFlush also calls flush, pushing pending SQL to the database immediately, which helps when you need a constraint error or trigger result now. The commit still happens at the end of the transaction.
03

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.
04

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

JPA and HibernateEasy
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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.
05

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.
06

Describe a time you disagreed with a teammate during a code review.

Collections and StreamsMedium
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Situation: a teammate used nested loops over two large lists to match records. Task: I felt it would be slow at production volume. Action: instead of just rejecting it, I wrote a quick benchmark showing the HashMap-based approach was much faster, and explained it calmly in a call. Result: he agreed, we merged the faster version, and we added a team guideline to back performance comments with data.
07

Why does React need a key prop when rendering lists?

React FrontendEasy
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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.
08

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

JPA and HibernateEasy
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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.
09

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.
10

What is the difference between a clustered and a non-clustered index?

SQL and DatabasesMedium
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A clustered index defines the physical order of rows, so the table data itself is stored in the index and there can be only one. In MySQL InnoDB the primary key is the clustered index. A non-clustered or secondary index is a separate structure holding the indexed columns and a pointer, or the primary key in InnoDB, to the row. That is why a short primary key keeps every secondary index smaller.
11

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.
12

What is the difference between localStorage, sessionStorage and cookies?

JavaScript, HTML and CSSMedium
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localStorage keeps data with no expiry for the origin, around 5 to 10 MB, and is shared across tabs. sessionStorage is similar but cleared when the tab closes and is per tab. Cookies are small, around 4 KB, are sent automatically with every request to the server and can be marked HttpOnly, Secure and SameSite. Use cookies for auth sessions, and storage for non-sensitive UI preferences only, since scripts can read storage.
13

What are fail-fast and fail-safe iterators?

Collections and StreamsMedium
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Fail-fast iterators, used by ArrayList and HashMap, track a modification count and throw ConcurrentModificationException if the collection is structurally changed during iteration, except through the iterator's own remove. Fail-safe or weakly consistent iterators, used by CopyOnWriteArrayList and ConcurrentHashMap, work on a snapshot or tolerate concurrent changes and never throw that exception, but they may not show the latest updates.
14

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.
15

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.
16

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.
17

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.
18

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.
19

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

JavaScript, HTML and CSSEasy
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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.
20

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.
21

Write code that runs tasks in parallel using ExecutorService and collects results.

Core JavaMedium
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ExecutorService pool = Executors.newFixedThreadPool(4); try { List<Callable<Integer>> tasks = List.of( () -> fetchCount("orders"), () -> fetchCount("users")); List<Future<Integer>> results = pool.invokeAll(tasks); for (Future<Integer> f : results) { System.out.println(f.get()); } } finally { pool.shutdown(); } // On Java 21 you can use Executors.newVirtualThreadPerTaskExecutor() // for I/O-bound tasks.
22

What is the difference between JDK, JRE and JVM?

Core JavaEasy
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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.
23

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.
24

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.
25

Users must upload files up to 200 MB. How do you design the upload API?

REST APIs, Security and MicroservicesMedium
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Avoid streaming large files through the application server. Generate a pre-signed URL for object storage like S3 or Azure Blob so the browser uploads directly, then the client calls the API to confirm and save the metadata. If uploads must go through Spring, raise spring.servlet.multipart.max-file-size, stream the input to storage instead of loading it into memory, validate type and size, and scan files for malware before use.
26

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.
27

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.
28

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.
29

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

Collections and StreamsEasy
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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.
30

Which features from Java 17 and Java 21 are useful for a full stack developer?

Core JavaMedium
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Java 17 brought records for immutable data carriers, sealed classes to restrict inheritance, text blocks for multi-line JSON or SQL, and pattern matching for instanceof. Java 21, the current widely used LTS alongside 17, added virtual threads for cheap blocking concurrency, pattern matching for switch, record patterns and sequenced collections. Spring Boot 3 needs Java 17 or later, and from Boot 3.2 virtual threads can be enabled with one property.

Frequently asked questions

What happens in the Java Full Stack with React telephonic interview round?

The telephonic or screening round is a 15-30 minute call with a recruiter or engineer. It checks your basics, your CV and whether you can explain Java Full Stack with React concepts clearly without a screen to point at.

How do I clear the telephonic interview round?

Keep your CV in front of you and be ready to explain every line on it. Answer in short, clear sentences; there is no whiteboard, so structure matters. Take the call somewhere quiet and confirm the time in advance.

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 questionsTechnical Round questionsManagerial Round questionsRapid-Fire questionsHR & behavioural questionsFull mock interview

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