Java Developer Interview Questions India 2026: Core Java, Spring Boot, and System Design

Core Java interview must-know concepts: (1) OOP fundamentals: abstraction, encapsulation, inheritance, polymorphism with real examples. Interviewers test with: 'Can you overload the main method in Java?' (yes, but JVM calls the String[] version as entry point), 'What is the difference between method overloading and overriding?' (compile-time vs runtime polymorphism). (2) Java memory model: stack vs heap, garbage collection (G1GC vs ZGC), memory leaks, strong/weak/soft/phantom references. (3) Collections framework: HashMap internals (how put() works, hash collisions, load factor, why HashMap capacity is a power of 2), ArrayList vs LinkedList vs ArrayDeque performance trade-offs, ConcurrentHashMap vs HashMap. (4) Multithreading: synchronized vs volatile vs Lock, deadlock conditions and prevention, thread pool (Executors.newFixedThreadPool vs newCachedThreadPool), CompletableFuture for async tasks. (5) Java 8+ features: streams (intermediate vs terminal operations), lambda expressions, Optional, functional interfaces (Function, Predicate, Consumer, Supplier), default methods in interfaces.

Spring Boot questions at product companies: (1) 'What is inversion of control and how does Spring implement it?' (IoC container manages object lifecycle; Spring uses dependency injection via @Autowired, constructor injection, or setter injection). (2) 'What is the difference between @Component, @Service, @Repository, and @Controller?' (all are stereotype annotations; @Repository adds database exception translation). (3) 'How does Spring Boot auto-configuration work?' (spring.factories scans the classpath and configures beans based on presence of specific classes). (4) 'What is the difference between @RestController and @Controller?' (@RestController = @Controller + @ResponseBody; returns JSON/XML directly). (5) 'How do you handle database transactions in Spring Boot?' (@Transactional annotation, propagation levels: REQUIRED vs REQUIRES_NEW vs NESTED, rollback rules). (6) 'What is the N+1 problem in JPA and how do you fix it?' (fetching a list of entities then issuing a separate query per entity for a relationship; fix with JOIN FETCH or @BatchSize).

Common Java coding questions in Indian interviews: (1) Easy-medium: 'Reverse a linked list iteratively and recursively.' 'Find all duplicates in an array using O(n) time and O(1) extra space' (mark elements by negating value at the index). 'Implement a LRU cache' (LinkedHashMap with removeEldestEntry or a HashMap plus doubly linked list). (2) String manipulation: 'Check if two strings are anagrams', 'Find the longest substring without repeating characters', 'Group anagrams from a list of strings'. (3) Collections and generics: 'Sort a list of employees by salary descending, then by name ascending' (Comparator chaining). 'Count word frequency in a list of strings using streams' (Collectors.groupingBy + Collectors.counting). (4) Concurrency: 'Implement a thread-safe bounded blocking queue' (PriorityBlockingQueue or a custom implementation with wait/notify). (5) Design: 'Design a parking lot in Java with OOP principles' (evaluates abstraction and class hierarchy design).

Java interview differences in India: Service companies (TCS, Infosys, Wipro, Cognizant): Core Java is tested to a moderate level (OOP, collections basics, exception handling). Spring Boot is rarely tested in depth for freshers. SQL and DBMS (joins, normalization, indexing basics) are a significant part of the interview. The bar for algorithmic coding is low (easy LeetCode difficulty). Product companies (Razorpay, Swiggy, Google India, Amazon India, Atlassian): Core Java depth (multithreading, Java memory model, generics, JVM internals for senior roles). Spring Boot depth (internals, how auto-configuration works, security, database transactions). LeetCode-style coding (medium to hard difficulty for senior roles; easy to medium for fresher campus roles). System design for mid-senior roles. Behavioural round (STAR-format stories about your past work). Preparation recommendation: for service companies, prioritise OOP, DBMS, and easy LeetCode (30-50 problems). For product companies, prioritise multithreading, Spring Boot internals, and 150-200 LeetCode problems (easy and medium).