Java Developer Interview Questions India 2026

Tip: `==` compares object references (memory addresses). `.equals()` compares logical content — overridden in String and most value classes. Classic trap: `new String("a") == new String("a")` is false; `.equals()` is true.

Tip: Lambda expressions, Stream API, Optional class, functional interfaces, default/static interface methods, new Date-Time API (java.time), and CompletableFuture. Streams and lambdas are the most commonly tested.

Tip: ArrayList: O(1) random access, O(n) insert/delete at middle. LinkedList: O(n) access, O(1) insert/delete at head/tail. Use ArrayList by default; LinkedList only when you frequently add/remove at ends and never random-access.

Tip: Checked exceptions (extend Exception) must be declared or caught — for recoverable conditions like file not found. Unchecked (extend RuntimeException) are for programming errors (NPE, ArrayIndexOutOfBounds). Do not catch RuntimeException broadly.

Tip: S: Single Responsibility — one class, one reason to change. O: Open/Closed — extend via inheritance, not modification. L: Liskov Substitution — subclasses must be substitutable. I: Interface Segregation — small focused interfaces. D: Dependency Inversion — depend on abstractions.

Tip: `synchronized` acquires a monitor lock — prevents concurrent access to a block. `volatile` ensures a variable is read from main memory, not a thread-local cache — no atomicity guarantee. Use `volatile` for single-write flags; `synchronized` for compound operations.

Tip: @SpringBootApplication triggers @EnableAutoConfiguration. Spring Boot scans META-INF/spring.factories, finds @Configuration classes, and applies them conditionally using @ConditionalOnClass/@ConditionalOnMissingBean. The app gets sensible defaults without XML.

Tip: Name a specific technical choice: service layer separation, DTO pattern, exception handler with @ControllerAdvice, custom validation. Tie it to the problem it solved, not just "it follows best practices."

Tip: Immediate: increase heap (-Xmx), take heap dump (jmap), restart if needed. Analysis: load heap dump in Eclipse MAT or VisualVM, find largest retained set. Common causes: unbounded caches, session leaks, large result sets loaded into memory. Fix root cause.

Tip: GC automatically reclaims unused heap memory. Generational GC: most objects die young (Eden). Young Gen: minor GC, fast. Old Gen: major GC, slower. G1GC (Java 9+ default) divides heap into regions for predictable pause times.

Tip: HashMap is non-synchronised (faster, not thread-safe), allows one null key. HashTable is synchronised (thread-safe but slow), no null keys. Prefer ConcurrentHashMap over HashTable in multithreaded code.

Tip: Java: JVM JIT compilation, mature concurrency primitives, type safety. Python: GIL limits true parallelism, but async frameworks (FastAPI) excel for I/O-bound workloads. Show you understand the trade-off — neither is universally better.