Java interview questions

Differentiate between JDK, JRE, JVM.

Easy · Java, Core Java, JVM

Why is Java called platform-independent?

Easy · Java, Core Java, JVM

What is a classloader in Java? Explain its hierarchy (Bootstrap, Extension, Application).

Medium · Java, Core Java, JVM

Differentiate between instance variable, class variable, and local variable.

Easy · Java, Core Java

Difference between == and .equals(). When should each be used?

Easy · Java, Core Java

Difference between String, StringBuilder, StringBuffer. Which one to use in multi-threaded scenarios?

Easy · Java, Core Java, Strings

Why are Strings immutable in Java? Explain design reasons.

Easy · Java, Core Java, Strings

What is String interning? How does it improve performance?

Medium · Java, Core Java, Strings

Differentiate between final, finally, finalize.

Easy · Java, Core Java

Differentiate between checked and unchecked exceptions. Provide examples.

Easy · Java, Core Java, Exception Handling

Difference between throw and throws keywords.

Easy · Java, Core Java, Exception Handling

What is try-with-resources? Why is it preferred over finally for closing resources?

Medium · Java, Core Java, Exception Handling

What is autoboxing and unboxing in Java? Provide examples.

Easy · Java, Core Java

Differentiate between primitive types and wrapper classes.

Easy · Java, Core Java

Can we make main() method final, static, synchronized, or overloaded?

Easy · Java, Core Java

How do annotations work in Java? How do you create custom annotations?

Medium · Java, Core Java

Differentiate between reflection and introspection.

Hard · Java, Core Java

How does dynamic class loading work in Java?

Hard · Java, Core Java, JVM

What is the role of Class.forName()?

Medium · Java, Core Java, JVM

What is the transient keyword? Why use it?

Easy · Java, Core Java

Difference between Serialization and Externalization.

Medium · Java, Core Java

How do you design an immutable class? Best practices?

Medium · Java, Core Java

Is Java pass-by-value or pass-by-reference? Explain with examples.

Easy · Java, Core Java

Differentiate between static initializer blocks and instance initializer blocks.

Easy · Java, Core Java

What are enums in Java? Can enums have methods, constructors, and fields? Provide examples.

Medium · Java, Core Java

What are EnumSet and EnumMap? Why are they more efficient than HashSet/HashMap for enums?

Medium · Java, Core Java, Collections

What are varargs in Java? What are the rules and pitfalls when using them?

Easy · Java, Core Java

Differentiate between static nested class, inner class, local class, and anonymous class.

Medium · Java, Core Java, OOP

What are generics in Java? Explain type erasure and why it was introduced.

Medium · Java, Core Java

Explain bounded wildcards in generics: <? extends T> vs <? super T>. What is the PECS principle?

Hard · Java, Core Java

What are covariance and contravariance in Java generics? Why is List<Dog> not a List<Animal>?

Hard · Java, Core Java

What is ThreadLocal? When and why should you use it? What are its pitfalls?

Medium · Java, Core Java, Concurrency

Difference between Thread.sleep() and Object.wait(). When to use which?

Easy · Java, Core Java, Concurrency

Difference between notify() and notifyAll(). When is each appropriate?

Medium · Java, Core Java, Concurrency

What is the volatile keyword in Java? How does it ensure visibility without atomicity?

Medium · Java, Core Java, Concurrency

Why is volatile necessary in double-checked locking for Singleton?

Hard · Java, Core Java, Concurrency

What are the four pillars of OOP (Encapsulation, Inheritance, Polymorphism, Abstraction)? Explain each with real-world examples.

Easy · Java, OOP

Difference between abstraction and encapsulation.

Easy · Java, OOP

Explain encapsulation with a practical code snippet.

Easy · Java, OOP, Encapsulation

Define coupling and cohesion in OOPs. Why is low coupling and high cohesion desirable?

Medium · Java, OOP, Design Principles

Differentiate between HAS-A and IS-A relationships with examples.

Easy · Java, OOP, Inheritance

Differentiate between composition and aggregation. Which is preferred and why?

Medium · Java, OOP, Design Principles

Difference between deep copy and shallow copy.

Medium · Java, OOP

Differentiate between clone() and copy constructor.

Medium · Java, OOP

Differentiate between Abstract Class and Interface. When should you use which?

Medium · Java, OOP

Can an abstract class have a constructor? Why?

Easy · Java, OOP

What is a marker interface? Give examples (e.g., Serializable, Cloneable).

Easy · Java, OOP, Interfaces

Can an interface have default and static methods (Java 8+ feature)? Provide use cases.

Medium · Java, OOP, Interfaces

What are default and static methods in interfaces? Why were they introduced?

Easy · Java, OOP, Interfaces

Can an interface extend multiple interfaces?

Easy · Java, OOP, Interfaces

Explain multiple inheritance in Java and how Java resolves conflicts.

Medium · Java, OOP, Interfaces

Why doesn't Java support multiple inheritance with classes?

Medium · Java, OOP

Why does Java support single inheritance for classes but multiple inheritance for interfaces?

Medium · Java, OOP, Interfaces

Explain the diamond problem. How Java 8 resolved it with default methods?

Hard · Java, OOP, Interfaces

What happens if two interfaces define the same default method?

Hard · Java, OOP, Interfaces

Differentiate between method overloading and method overriding with examples.

Easy · Java, OOP, Polymorphism

What are the rules of method overriding (return type, access modifiers, exceptions)?

Medium · Java, OOP, Polymorphism

Can we override private, static, or final methods? Why or why not?

Medium · Java, OOP, Polymorphism

What is dynamic method dispatch in Java? Provide an example.

Medium · Java, OOP, Polymorphism

Differentiate between compile-time polymorphism and runtime polymorphism.

Easy · Java, OOP, Polymorphism

Provide a real-world example of polymorphism in Java.

Easy · Java, OOP, Polymorphism

Can a constructor be overridden? Why or why not?

Easy · Java, OOP

What is object slicing in OOPs?

Medium · Java, OOP

Difference between deep inheritance vs shallow inheritance.

Medium · Java, OOP

What are the commonly overridden methods from Object class (equals, hashCode, toString, clone)? How do you override them correctly?

Medium · Java, OOP

Explain SOLID, DRY, KISS, YAGNI principles in Java OOP design.

Medium · Java, OOP, Design Principles

Which functional programming principles did Java 8 adopt?

Medium · Java, Java 8+, Lambda

What are Lambda expressions? Provide syntax and real-world examples.

Easy · Java, Java 8+, Lambda

What are functional interfaces? Provide examples (Runnable, Supplier, Consumer, Function).

Easy · Java, Java 8+, Lambda

Differentiate between Predicate, Function, Consumer, Supplier.

Medium · Java, Java 8+, Lambda

What are method references? What are their types?

Easy · Java, Java 8+, Lambda

Difference between stream() and parallelStream(). When to use which?

Medium · Java, Java 8+, Streams

Differentiate between intermediate and terminal stream operations with examples.

Medium · Java, Java 8+, Streams

Explain the difference between map(), flatMap(), filter(), reduce() in Streams.

Medium · Java, Java 8+, Streams

Difference between flatMap in Streams vs Optional.

Medium · Java, Java 8+, Streams

How does collect() work? Explain common collectors (toList, toSet, joining, groupingBy, partitioningBy).

Medium · Java, Java 8+, Streams

Explain Collectors.groupingBy() and Collectors.partitioningBy() with examples.

Medium · Java, Java 8+, Streams

Difference between findFirst() and findAny().

Easy · Java, Java 8+, Streams

Difference between forEach() and forEachOrdered().

Easy · Java, Java 8+, Streams

Can a stream be reused? Why not?

Easy · Java, Java 8+, Streams

Difference between peek() and map().

Easy · Java, Java 8+, Streams

What is a short-circuiting operation in streams? Examples.

Medium · Java, Java 8+, Streams

What is a Spliterator? How is it different from Iterator?

Hard · Java, Java 8+, Streams

Difference between reduce() and collect().

Medium · Java, Java 8+, Streams

When should we use parallelStream()? What are the risks?

Hard · Java, Java 8+, Streams

How does stream pipeline execution work internally?

Hard · Java, Java 8+, Streams

Difference between mapMulti() (Java 16) and flatMap().

Hard · Java, Java 8+, Streams

What is Optional in Java? How does it help avoid NullPointerException?

Easy · Java, Java 8+

Difference between Optional.of(), Optional.ofNullable(), Optional.empty().

Easy · Java, Java 8+

How do you safely retrieve a nested object's field using Optional?

Medium · Java, Java 8+

How do you return a default value if null using orElse() and orElseGet()?

Easy · Java, Java 8+

How do you throw an exception if a value is not present using orElseThrow()?

Easy · Java, Java 8+

What is the difference between map() and flatMap() in Optional?

Medium · Java, Java 8+

What are the best practices when using Optional in APIs?

Medium · Java, Java 8+

Differentiate between LocalDate, LocalTime, LocalDateTime, ZonedDateTime.

Easy · Java, Java 8+, Date & Time

Compare Date/Calendar API with java.time API. Why is the new API better?

Medium · Java, Java 8+, Date & Time

Difference between List, Set, Map.

Easy · Java, Collections

Difference between ArrayList and LinkedList.

Easy · Java, Collections

Difference between HashSet, LinkedHashSet, TreeSet.

Easy · Java, Collections

Difference between HashMap, LinkedHashMap, TreeMap.

Easy · Java, Collections

Difference between HashMap and Hashtable.

Easy · Java, Collections, Concurrency

What happens internally when HashMap.put() is called?

Easy · Java, Collections

How does HashMap work internally? (hashCode, equals, buckets, rehashing, load factor).

Hard · Java, Collections

How does LinkedHashMap maintain insertion order?

Medium · Java, Collections

What is the time complexity of TreeMap for put()/get()?

Easy · Java, Collections

Difference between Comparable and Comparator.

Easy · Java, Collections

Differentiate between fail-fast and fail-safe iterators.

Medium · Java, Collections

Differentiate between Enumeration, Iterator, and ListIterator.

Easy · Java, Collections

What is a PriorityQueue and how does it work internally?

Medium · Java, Collections

What is an IdentityHashMap? Where is it used?

Hard · Java, Collections

How are WeakHashMap and SoftReference used in caching?

Hard · Java, Collections, JVM

What is a ConcurrentSkipListMap and when would you use it?

Hard · Java, Collections, Concurrency

How does ConcurrentHashMap handle collisions?

Hard · Java, Collections, Concurrency

What are Immutable Collections in Java 9+?

Easy · Java, Collections

Why is hashCode() important in collections?

Medium · Java, Collections

Why must equals() and hashCode() be consistent?

Medium · Java, Collections

What happens if you override equals() without overriding hashCode()?

Medium · Java, Collections

Compare the thread-safety of StringBuffer vs StringBuilder.

Easy · Java, Concurrency, Core Java

What is the copy-on-write mechanism?

Medium · Java, Concurrency

Difference between synchronized and ReentrantLock.

Hard · Java, Concurrency

What is a ReadWriteLock? When should you use it?

Medium · Java, Concurrency

What is the difference between StampedLock and ReentrantReadWriteLock?

Hard · Java, Concurrency

Can two threads access different synchronized methods on the same object simultaneously?

Medium · Java, Concurrency

How do you detect and resolve deadlocks in Java?

Hard · Java, Concurrency

What is the difference between livelock and starvation?

Medium · Java, Concurrency

What is the difference between ConcurrentHashMap and SynchronizedMap?

Medium · Java, Collections, Concurrency

How does ConcurrentHashMap achieve thread safety internally?

Hard · Java, Collections, Concurrency

What is the difference between BlockingQueue and ConcurrentLinkedQueue?

Medium · Java, Collections, Concurrency

Explain the producer-consumer problem using BlockingQueue.

Medium · Java, Collections, Concurrency

What is the difference between CopyOnWriteArrayList and ArrayList?

Medium · Java, Collections, Concurrency

How does ConcurrentSkipListMap ensure thread safety?

Hard · Java, Collections, Concurrency

What is the ForkJoin framework? Provide a use case.

Hard · Java, Concurrency

What are Phaser and Exchanger in Java concurrency utilities?

Hard · Java, Concurrency

What is ThreadPoolExecutor? Explain its parameters.

Hard · Java, Concurrency

Difference between newFixedThreadPool(), newCachedThreadPool(), and newSingleThreadExecutor().

Medium · Java, Concurrency

What is backpressure in concurrent systems?

Hard · Java, Concurrency

What happens if you submit too many tasks to a thread pool?

Medium · Java, Concurrency

How do you implement a rate limiter in Java?

Hard · Java, Concurrency

Explain the difference between thenApply() and thenCompose() in CompletableFuture.

Hard · Java, Concurrency, Java 8+

Explain JVM Architecture (ClassLoader, Method Area, Heap, Stack, Native Method Area, PC Registers).

Medium · Java, Memory & JVM, JVM

Differentiate between stack memory and heap memory.

Easy · Java, Memory & JVM

Difference between OutOfMemoryError and StackOverflowError.

Easy · Java, Memory & JVM, JVM

What is garbage collection? How does it work?

Medium · Java, Memory & JVM, JVM

What are GC algorithms (Serial, Parallel, CMS, G1)?

Hard · Java, Memory & JVM, JVM

Compare G1, ZGC, and Shenandoah garbage collectors.

Hard · Java, Memory & JVM, JVM

What are stop-the-world events in GC?

Medium · Java, Memory & JVM, JVM

Explain strong, weak, soft, and phantom references with use cases.

Hard · Java, Memory & JVM, JVM

Differentiate between weak references and phantom references.

Hard · Java, Memory & JVM, JVM

Differentiate between JIT compiler and interpreter.

Medium · Java, Memory & JVM, JVM

What is escape analysis in JVM optimizations?

Hard · Java, Memory & JVM, JVM

What is Metaspace (Java 8+) vs PermGen (Java 7-)?

Medium · Java, Memory & JVM, JVM

Tools to monitor JVM memory (jconsole, jvisualvm).

Easy · Java, Memory & JVM, JVM

How do you detect and fix memory leaks in Java?

Hard · Java, Memory & JVM, JVM

Difference between compile-time constants (final static) and runtime constants.

Medium · Java, Tricky, Core Java

Why must HashMap keys be immutable? What goes wrong if they are mutable?

Medium · Java, Tricky, Collections

Why is String commonly used as a key in HashMap?

Easy · Java, Tricky, Collections

What happens if hashCode() is not overridden properly?

Medium · Java, Tricky, Collections

Difference between ConcurrentModificationException and IllegalStateException.

Medium · Java, Tricky, Collections

What is the impact of load factor tuning in HashMap?

Hard · Java, Tricky, Collections

When to prefer LinkedList over ArrayList?

Medium · Java, Tricky, Collections

Real-world cases where CopyOnWriteArrayList is preferred?

Medium · Java, Tricky, Collections

How to handle large datasets efficiently with Streams?

Hard · Java, Tricky, Streams

When should we not use parallel streams?

Medium · Java, Tricky, Streams

Can a Stream run infinitely? How do you handle it?

Medium · Java, Tricky, Streams

Why prefer CompletableFuture over Thread or Executor?

Medium · Java, Tricky, Concurrency

What are the pitfalls of using Optional as a field in entities?

Hard · Java, Tricky, Java 8+

How to debug a thread deadlock in a production JVM?

Hard · Java, Tricky, Concurrency

What is the Java Memory Model (JMM)? Explain happens-before relationship.

Hard · Java, Advanced Java, Concurrency

How do AtomicInteger, AtomicLong, and AtomicReference work internally? What is CAS (Compare-And-Swap)?

Hard · Java, Advanced Java, Concurrency

Differentiate between CountDownLatch, CyclicBarrier, and Semaphore with use cases.

Hard · Java, Advanced Java, Concurrency

What is false sharing in CPU caches? How does it impact Java concurrent applications?

Hard · Java, Advanced Java, Concurrency

What is lock-free programming? How does it improve concurrency compared to synchronized blocks?

Hard · Java, Advanced Java, Concurrency

Classify the 23 GoF design patterns (Creational, Structural, Behavioral). Give two examples from each.

Medium · Java, Advanced Java, Design Patterns

Implement Singleton using: Eager Loading, Lazy Loading, Bill Pugh, Enum, and Double-Checked Locking. Compare pros/cons.

Hard · Java, Advanced Java, Design Patterns

Differentiate between Factory Method and Abstract Factory patterns. When to use which?

Medium · Java, Advanced Java, Design Patterns

Explain the Builder pattern. Why is it preferred over telescoping constructors?

Easy · Java, Advanced Java, Design Patterns

Explain the Strategy pattern. How does Java 8 simplify it with lambdas?

Medium · Java, Advanced Java, Design Patterns

Explain the Observer pattern. How is it implemented in Java? How does it relate to event listeners?

Medium · Java, Advanced Java, Design Patterns

Differentiate between Decorator and Proxy patterns. Provide real-world examples.

Medium · Java, Advanced Java, Design Patterns

Explain the Template Method pattern. Where is it used in Java frameworks (e.g., JdbcTemplate)?

Medium · Java, Advanced Java, Design Patterns

Differentiate between Adapter and Facade patterns with Java examples.

Medium · Java, Advanced Java, Design Patterns

Explain the Chain of Responsibility pattern. Where is it used in Java (servlet filters, logging)?

Medium · Java, Advanced Java, Design Patterns

Explain CompletableFuture chaining: thenApply, thenAccept, thenRun, handle, exceptionally.

Hard · Java, Advanced Java, Concurrency

Difference between Future and CompletableFuture. What limitations does Future have?

Medium · Java, Advanced Java, Concurrency

What are method handles (java.lang.invoke)? How do they differ from reflection?

Hard · Java, Advanced Java, JVM

How does invokedynamic work under the hood? How are Java lambdas compiled?

Hard · Java, Advanced Java, JVM

What is off-heap memory in Java? When and why would you use it?

Hard · Java, Advanced Java, Memory & JVM

What are the major features introduced in Java 17 LTS?

Easy · Java, Java 17+

What are the major features introduced in Java 21 LTS (virtual threads, sequenced collections, record patterns)?

Easy · Java, Java 17+

What are Records in Java 16+? How do they differ from regular POJOs? What are their limitations?

Easy · Java, Java 17+

What is a compact constructor in a Java Record? When would you use it?

Medium · Java, Java 17+

What are Sealed Classes in Java 17? What problem do they solve? How do they work with pattern matching?

Medium · Java, Java 17+

What are Sealed Interfaces? How do they enable exhaustive type hierarchies?

Medium · Java, Java 17+

What is Pattern Matching for instanceof (Java 16)? How does it reduce boilerplate?

Easy · Java, Java 17+

What is Pattern Matching for Switch (Java 21)? How do guard patterns work?

Medium · Java, Java 17+

What are Record Patterns (Java 21)? How do they enable deconstruction in pattern matching?

Medium · Java, Java 17+

What are Switch Expressions (Java 14)? How do they differ from switch statements? Explain arrow syntax and yield.

Easy · Java, Java 17+

What are Text Blocks (Java 15)? How do they handle indentation and escape sequences?

Easy · Java, Java 17+

What are Virtual Threads (Project Loom, Java 21)? How do they differ from platform threads?

Hard · Java, Java 17+, Concurrency

How do virtual threads compare to reactive programming (WebFlux)? When would you choose one over the other?

Hard · Java, Java 17+, Concurrency

What is Structured Concurrency (Java 21)? How does it simplify concurrent task management?

Hard · Java, Java 17+, Concurrency

What are Sequenced Collections (Java 21)? What interfaces were introduced and why?

Easy · Java, Java 17+, Collections

What are unnamed patterns and variables (Java 21)? How does the underscore _ work in pattern matching?

Medium · Java, Java 17+

What is the Foreign Function & Memory API (Java 22)? How does it replace JNI?

Hard · Java, Java 17+, Advanced Java

What are Scoped Values (Java 21)? How do they differ from ThreadLocal?

Hard · Java, Java 17+, Concurrency

How do Java Records compare to Lombok @Value? What are the trade-offs?

Medium · Java, Java 17+

How do Sealed Classes + Pattern Matching together enable Algebraic Data Types (ADTs) in Java?

Hard · Java, Java 17+

Explain the Java Exception hierarchy. What is the difference between Error and Exception?

Easy · Java, Core Java, Exception Handling

What is multi-catch exception handling (Java 7)? What are its benefits?

Easy · Java, Core Java, Exception Handling

What is the execution order of try-catch-finally? What if both catch and finally throw exceptions?

Medium · Java, Core Java, Exception Handling

How does the Iterable interface work? How does for-each loop use it internally?

Easy · Java, Core Java, Collections

What is the difference between ArrayList and Arrays.asList()? What are the pitfalls of Arrays.asList()?

Medium · Java, Core Java, Collections

Differentiate between Collections.unmodifiableList(), List.of(), and Collections.singletonList().

Medium · Java, Core Java, Collections

What is the @FunctionalInterface annotation? What happens if you add a second abstract method?

Easy · Java, Core Java, Java 8+

Why is string concatenation using + in a loop inefficient? How does the compiler optimize it?

Easy · Java, Core Java, Strings

What is the String pool? How does String.intern() work? What are the memory implications?

Medium · Java, Core Java, Strings

Can we catch Error in Java? Should we? When might catching OutOfMemoryError be justified?

Medium · Java, Core Java, Exception Handling

After Java 8 default methods, when should you prefer an abstract class over an interface?

Medium · Java, OOP, Java 8+

What is the diamond operator <> introduced in Java 7? How does it support type inference?

Easy · Java, Core Java

What is the difference between a synchronized method and a synchronized block? Which is preferred?

Medium · Java, Concurrency

What is an intrinsic lock (monitor) in Java? How does it relate to synchronized?

Medium · Java, Concurrency

What is Thread.interrupt()? How do you correctly handle interruption in long-running tasks?

Medium · Java, Concurrency

What are daemon threads in Java? How do they differ from user threads?

Easy · Java, Concurrency

Difference between Callable and Runnable. When would you prefer Callable?

Easy · Java, Concurrency

What is the difference between ExecutorService.submit() and execute()? What does submit() return?

Medium · Java, Concurrency

What is the difference between CompletableFuture.allOf() and anyOf()? Provide use cases.

Medium · Java, Concurrency, Java 8+

What is thread contention? How do you detect and reduce it in a Java application?

Hard · Java, Concurrency

Why is Deque preferred over Stack in Java? What implementations does Deque have?

Easy · Java, Collections

What is the NavigableMap interface? What methods does it add over SortedMap?

Medium · Java, Collections

What is a BitSet in Java? When is it more memory-efficient than a boolean array?

Medium · Java, Collections

How and when does a HashMap resize? What is rehashing and what is its time complexity?

Hard · Java, Collections

Explain the Composite pattern. Where is it used in Java (e.g., file systems, UI trees)?

Medium · Java, Advanced Java, Design Patterns

Explain the Flyweight pattern. How does Java String pool use it?

Medium · Java, Advanced Java, Design Patterns

Explain the State pattern. How does it differ from Strategy? Provide a real-world Java example.

Medium · Java, Advanced Java, Design Patterns

Explain the Iterator pattern. How is it implemented in the Java Collections Framework?

Easy · Java, Advanced Java, Design Patterns

Explain the Command pattern. How can Java 8 lambdas simplify its implementation?

Medium · Java, Advanced Java, Design Patterns

What is var (local variable type inference, Java 10)? What are its rules and limitations?

Easy · Java, Java 17+

What are enhanced NullPointerException messages (Java 14)? How do they help with debugging?

Easy · Java, Java 17+

What is String.formatted() introduced in Java 15? How does it compare to String.format()?

Easy · Java, Java 17+, Strings

What are local records and local classes (Java 16+)? When would you define a record inside a method?

Medium · Java, Java 17+

How does pattern matching with instanceof handle complex conditions and null checks in Java 21?

Medium · Java, Java 17+

Can a Java Record have custom methods, implement interfaces, and be generic? Explain with examples.

Medium · Java, Java 17+

What are common JVM GC tuning flags (-Xmx, -Xms, -XX:+UseG1GC)? How do you choose GC settings for production?

Hard · Java, Memory & JVM, JVM

What are the phases of class loading in JVM (Loading, Linking, Initialization)? When does static initialization occur?

Hard · Java, Memory & JVM, JVM

What is the difference between Java IO and NIO? What are channels, buffers, and selectors in NIO?

Hard · Java, Advanced Java

How do you implement AutoCloseable to work with try-with-resources? Provide a practical example.

Medium · Java, Core Java, Exception Handling

What new String methods were introduced in Java 11 (isBlank, strip, repeat, lines)?

Easy · Java, Java 17+, Strings

How does Optional.stream() (Java 9) integrate Optional with the Stream API?

Medium · Java, Java 8+

What are collection factory methods (List.of, Set.of, Map.of) introduced in Java 9? How do they differ from Arrays.asList?

Easy · Java, Collections, Java 8+

What are private methods in interfaces (Java 9)? How do they help with code reuse in default methods?

Medium · Java, OOP, Interfaces

Explain the static and final keywords in Java. What can they modify and what are common use cases?

Easy · Java, Core Java, Fresher

What are the four access modifiers in Java? Explain their scope with package and inheritance examples.

Easy · Java, Core Java, OOP

How do you read and write files in Java? Explain FileReader, BufferedReader, InputStream, and try-with-resources.

Medium · Java, Core Java, Fresher

How do you write unit tests in Java using JUnit 5 and Mockito? Explain @Test, @Mock, when/then, and verify.

Easy · Java, Core Java, Senior

How does JDBC work in Java? Explain DriverManager, connection pooling (HikariCP), transactions, and isolation levels.

Hard · Java, Advanced Java, Senior

What are deserialization vulnerabilities in Java? How do you prevent insecure deserialization attacks?

Hard · Java, Advanced Java, Senior

How do you maintain code quality at scale in Java teams? Cover coding standards, reviews, static analysis, and technical debt.

Easy · Java, Core Java, Staff

What are best practices for designing Java APIs? Cover backward compatibility, deprecation, and semantic versioning.

Easy · Java, Advanced Java, Staff

How do Maven and Gradle manage Java dependencies? Explain BOMs, dependency conflicts, and version alignment.

Easy · Java, Core Java, Staff

How do you implement structured logging and observability in Java services? Cover MDC, correlation IDs, and metrics.

Easy · Java, Advanced Java, Staff

What is the Java Platform Module System (JPMS)? Explain module-info.java, exports, requires, and migration challenges.

Medium · Java, Advanced Java, Staff

What are OWASP Top 10 risks most relevant to Java applications? How do you apply secure coding practices?

Medium · Java, Advanced Java, Staff

How do you benchmark Java code with JMH? Explain warmup, fork, blackholes, and common pitfalls.

Medium · Java, Advanced Java, Memory & JVM

How do Spring and Hibernate use JDK dynamic proxies and CGLIB? Explain AOP proxy creation at runtime.

Medium · Java, Advanced Java, Staff

How do you analyze JVM heap dumps and thread dumps in production? What tools and patterns do you look for?

Medium · Java, Memory & JVM, Staff

How do you implement idempotency and saga patterns in Java microservices? Provide design and code-level guidance.

Hard · Java, Advanced Java, Staff

How do you design high-throughput Java services? Cover batching, async pipelines, queue sizing, and backpressure.

Hard · Java, Concurrency, Staff

Explain circuit breaker, bulkhead, retry, and rate limiter patterns in Java. When and how do you apply each?

Hard · Java, Advanced Java, Staff

How do you plan and execute a Java version migration (8 → 11 → 17 → 21)? Cover risks, testing, and rollback.

Hard · Java, Java 17+, Staff

How do you decide build vs buy and select Java frameworks/libraries? Explain evaluation criteria and team fit.

Hard · Java, Advanced Java, Staff

How do you design shared Java libraries and platform golden paths for multiple teams? Cover versioning and governance.

Hard · Java, Advanced Java, Staff

Explain the Java ecosystem: JDK vendors (Oracle, Temurin, GraalVM), LTS releases, and support lifecycle.

Easy · Java, Core Java, Architect

Compare monolith, modular monolith, microservices, and event-driven architectures from a Java platform perspective.

Easy · Java, Advanced Java, Architect

How do non-functional requirements (scalability, availability, latency, cost) drive Java architecture decisions?

Easy · Java, Advanced Java, Architect

How do you define service boundaries and API contracts in Java-based systems? Cover DTOs, versioning, and ownership.

Medium · Java, Advanced Java, Architect

What makes a Java application cloud-native? Cover containers, Kubernetes, 12-factor principles, and health probes.

Medium · Java, Advanced Java, Architect

How do you design caching layers and read/write splitting for Java data-intensive applications?

Medium · Java, Advanced Java, Architect

What integration patterns (Kafka, REST, gRPC) work best in Java ecosystems? How do you enforce contract testing?

Medium · Java, Advanced Java, Architect

How do you implement multi-tenancy in Java applications? Compare shared-schema, schema-per-tenant, and DB-per-tenant.

Medium · Java, Advanced Java, Architect

How do you design CI/CD pipelines for Java services? Cover blue-green, canary deployments, and feature flags.

Medium · Java, Advanced Java, Architect

What are GraalVM and native image compilation trade-offs for Java? Cover startup, memory, reflection, and AOT limits.

Hard · Java, Java 17+, Memory & JVM

How do you architect an internal Java platform SDK and shared runtime for dozens of services?

Hard · Java, Advanced Java, Architect

How do you create an org-wide Java LTS adoption roadmap and training plan? Cover governance and phased rollout.

Hard · Java, Java 17+, Architect

How do you model JVM capacity, memory footprint, and cost at scale? Cover autoscaling and FinOps for Java workloads.

Hard · Java, Memory & JVM, Architect

How do you design multi-region resilience and chaos engineering for Java-based distributed systems?

Hard · Java, Advanced Java, Architect

How do you govern Java architecture decisions across teams? Explain ADRs, standards enforcement, and exception processes.

Hard · Java, Advanced Java, Architect

How do you modernize legacy Java monoliths using the Strangler Fig pattern? Cover risk management and incremental migration.

Hard · Java, Advanced Java, Architect