πŸ† Ultimate Java 8+ & OOPs Interview Preparation Master List

Table of Contents
  1. OOPs Concepts (Core)
  2. Core Java – Fundamentals
  3. Java 8+ Features (Core)
  4. Multithreading & Concurrency (Core)
  5. Collections Framework (Core)
  6. Memory & JVM
  7. Advanced / Tricky Java Questions
  8. OOPs Concepts (Extended)
  9. Core Java (Advanced Concepts)
  10. Java 8+ Features (Advanced)
  11. Collections & Concurrency (Advanced)
  12. Coding Interview Questions (Java 8)
  13. Optional Usage (Java 8+)
  14. Concurrency Coding Challenges
  15. Collections Coding Challenges
  16. Tricky & Scenario-Based Questions
  17. Conclusion
1. OOPs Concepts (Core)
  1. What are the four pillars of OOP (Encapsulation, Inheritance, Polymorphism, Abstraction)? Explain each with real-world examples.
  2. Differentiate between Abstract Class and Interface. When should you use which?
  3. Can an interface have default and static methods (Java 8+ feature)? Provide use cases.
  4. Explain multiple inheritance in Java and how Java resolves conflicts.
  5. Differentiate between method overloading and method overriding with examples.
  6. What are the rules of method overriding (return type, access modifiers, exceptions)?
  7. What is dynamic method dispatch in Java? Provide an example.
  8. Differentiate between compile-time polymorphism and runtime polymorphism.
  9. Can a constructor be overridden? Why or why not?
  10. Differentiate between HAS-A and IS-A relationships with examples.
  11. Explain encapsulation with a practical code snippet.
  12. Define coupling and cohesion in OOPs. Why is low coupling and high cohesion desirable?
  13. What is a marker interface? Give examples (e.g., Serializable, Cloneable).
  14. Differentiate between composition and aggregation. Which is preferred and why?
  15. What are the commonly overridden methods from Object class (equals, hashCode, toString, clone)? How do you override them correctly?
2. Core Java – Fundamentals
  1. Differentiate between JDK, JRE, JVM.
  2. What is a classloader in Java? Explain its hierarchy (Bootstrap, Extension, Application).
  3. Difference between == and .equals(). When should each be used?
  4. Difference between String, StringBuilder, StringBuffer. Which one to use in multi-threaded scenarios?
  5. Why are Strings immutable in Java? Explain design reasons.
  6. What is String interning? How does it improve performance?
  7. Differentiate between final, finally, finalize.
  8. Can we override private, static, or final methods? Why or why not?
  9. Differentiate between checked and unchecked exceptions. Provide examples.
  10. Difference between throw and throws keywords.
  11. What is try-with-resources? Why is it preferred over finally for closing resources?
  12. What is autoboxing and unboxing in Java? Provide examples.
  13. Differentiate between primitive types and wrapper classes.
3. Java 8+ Features (Core)
  1. What are Lambda expressions? Provide syntax and real-world examples.
  2. What are functional interfaces? Provide examples (Runnable, Supplier, Consumer, Function).
  3. Differentiate between Predicate, Function, Consumer, Supplier.
  4. What are default and static methods in interfaces? Why were they introduced?
  5. Difference between stream() and parallelStream(). When to use which?
  6. Differentiate between intermediate and terminal stream operations with examples.
  7. Explain the difference between map(), flatMap(), filter(), reduce() in Streams.
  8. How does collect() work? Explain common collectors (toList, toSet, joining, groupingBy, partitioningBy).
  9. What is Optional in Java? How does it help avoid NullPointerException?
  10. Difference between findFirst() and findAny().
  11. Difference between forEach() and forEachOrdered().
  12. What are method references? What are their types?
  13. Differentiate between LocalDate, LocalTime, LocalDateTime, ZonedDateTime.
  14. Compare Date/Calendar API with java.time API. Why is the new API better?
4. Multithreading & Concurrency (Core)
  1. Difference between a process and a thread.
  2. Explain the lifecycle of a thread.
  3. Difference between Runnable and Callable.
  4. Differentiate between a synchronized method and a synchronized block.
  5. What is the volatile keyword? When should it be used?
  6. Difference between wait(), notify(), and notifyAll().
  7. Difference between sleep() and wait().
  8. Difference between ExecutorService and ForkJoinPool.
  9. Explain ConcurrentHashMap and CopyOnWriteArrayList.
  10. Difference between synchronized collections and concurrent collections.
  11. What is a deadlock? How can you detect and prevent it?
  12. Difference between Atomic classes and synchronized blocks.
  13. Explain CompletableFuture and its advantages over traditional threads.
  14. Differentiate between Semaphore, CountDownLatch, CyclicBarrier.
  15. What is ThreadLocal? Provide use cases.
  16. What is false sharing in multithreading? How do you prevent it?
  17. What is busy waiting? Why is it a performance issue?
  18. How do you prevent thread starvation?
5. Collections Framework (Core)
  1. Difference between List, Set, Map.
  2. Difference between HashMap, LinkedHashMap, TreeMap.
  3. Difference between HashSet, LinkedHashSet, TreeSet.
  4. Difference between ArrayList and LinkedList.
  5. Difference between HashMap and Hashtable.
  6. How does HashMap work internally? (hashCode, equals, buckets, rehashing, load factor).
  7. Differentiate between fail-fast and fail-safe iterators.
  8. Difference between Comparable and Comparator.
  9. What is a PriorityQueue and how does it work internally?
  10. Differentiate between Enumeration, Iterator, and ListIterator.
  11. What is an IdentityHashMap? Where is it used?
  12. How are WeakHashMap and SoftReference used in caching?
  13. What is a ConcurrentSkipListMap and when would you use it?
  14. How does LinkedHashMap maintain insertion order?
  15. What happens if you mutate a key after inserting it into a HashMap?
  16. Why must HashMap keys be immutable?
  17. Why is load factor important in HashMap?
  18. What is the time complexity of TreeMap for put()/get()?
  19. How does ConcurrentHashMap handle collisions?
6. Memory & JVM
  1. Explain JVM Architecture (ClassLoader, Method Area, Heap, Stack, Native Method Area, PC Registers).
  2. What is garbage collection? How does it work?
  3. What are GC algorithms (Serial, Parallel, CMS, G1)?
  4. Explain strong, weak, soft, and phantom references with use cases.
  5. Differentiate between stack memory and heap memory.
  6. Difference between OutOfMemoryError and StackOverflowError.
  7. How do you create immutable classes? Provide best practices.
  8. What is escape analysis in JVM optimizations?
  9. Differentiate between JIT compiler and interpreter.
  10. What is Metaspace (Java 8+) vs PermGen (Java 7-)?
  11. What are stop-the-world events in GC?
  12. Tools to monitor JVM memory (jconsole, jvisualvm).
  13. How do you detect and fix memory leaks in Java?
7. Advanced / Tricky Java Questions
  1. Why doesn’t Java support multiple inheritance with classes?
  2. Difference between deep copy and shallow copy.
  3. Differentiate between clone() and copy constructor.
  4. What is the transient keyword? Why use it?
  5. Difference between Serialization and Externalization.
  6. Why is hashCode() important in collections?
  7. Why must equals() and hashCode() be consistent?
  8. Difference between synchronized and ReentrantLock.
  9. Why is Java called platform-independent?
  10. Difference between compile-time constants (final static) and runtime constants.
  11. Why is String commonly used as a HashMap key?
  12. What happens if you override equals() without overriding hashCode()?
  13. Differentiate between ConcurrentModificationException and IllegalStateException.
8. OOPs Concepts (Extended)
  1. Difference between abstraction and encapsulation.
  2. Can an abstract class have a constructor? Why?
  3. Can an interface extend multiple interfaces?
  4. Why does Java support single inheritance for classes but multiple inheritance for interfaces?
  5. Explain the diamond problem. How Java 8 resolved it with default methods?
  6. What happens if two interfaces define the same default method?
  7. What is object slicing in OOPs?
  8. Difference between deep inheritance vs shallow inheritance.
  9. Provide a real-world example of polymorphism in Java.
  10. Explain SOLID, DRY, KISS, YAGNI principles in Java OOP design.
9. Core Java (Advanced Concepts)
  1. Differentiate between instance variable, class variable, and local variable.
  2. How do you design an immutable class? Best practices?
  3. What is the copy-on-write mechanism?
  4. Which functional programming principles did Java 8 adopt?
  5. Differentiate between reflection and introspection.
  6. How does dynamic class loading work in Java?
  7. What is the role of Class.forName()?
  8. Can we make main() method final, static, synchronized, or overloaded?
  9. How do annotations work in Java? How do you create custom annotations?
  10. Differentiate between weak references and phantom references.
10. Java 8+ Features (Advanced)
  1. Explain Collectors.groupingBy() and Collectors.partitioningBy() with examples.
  2. Difference between flatMap in Streams vs Optional.
  3. Can a stream be reused? Why not?
  4. Difference between peek() and map().
  5. What is a short-circuiting operation in streams? Examples.
  6. What is a Spliterator? How is it different from Iterator?
  7. Difference between reduce() and collect().
  8. When should we use parallelStream()? What are the risks?
  9. How does stream pipeline execution work internally?
  10. Difference between Optional.of(), Optional.ofNullable(), Optional.empty().
  11. Best practices when using Optional in large projects.
  12. Difference between mapMulti() (Java 16) and flatMap().
11. Collections & Concurrency (Advanced)
  1. Difference between ConcurrentHashMap and SynchronizedMap.
  2. How does ConcurrentHashMap achieve thread safety? (Segments, locks, CAS).
  3. Difference between BlockingQueue and ConcurrentLinkedQueue.
  4. Explain producer-consumer problem using BlockingQueue.
  5. Differentiate between CopyOnWriteArrayList and ArrayList.
  6. What are Immutable Collections in Java 9+?
  7. How do you detect and resolve deadlocks?
  8. What is the difference between livelock and starvation?
  9. Compare thread-safety of StringBuffer vs StringBuilder.
  10. What is the ForkJoin framework? Provide a use case.
  11. Can two threads access different synchronized methods on the same object simultaneously?
  12. What is a ReadWriteLock? When should you use it?
  13. Difference between StampedLock and ReentrantReadWriteLock.
  14. How does ConcurrentSkipListMap ensure thread safety?
  15. What are Phaser and Exchanger in concurrency utilities?
  16. What is ThreadPoolExecutor? Explain its parameters (corePoolSize, maxPoolSize, queue, etc.).
  17. Difference between Executors.newFixedThreadPool(), newCachedThreadPool(), newSingleThreadExecutor().
  18. What is BackPressure in concurrent systems?
  19. What happens if you submit too many tasks to a thread pool?
  20. How do you implement a rate limiter in Java?
12. Coding Interview Questions (Java 8)
  1. Find the first non-repeating character in a String using Java 8 Streams.
  2. Find duplicate elements in a list using Streams.
  3. Count occurrences of each word in a sentence (word frequency map).
  4. Sort a Map by values using Streams.
  5. Find the second highest/lowest element in a list.
  6. Reverse a String using Streams.
  7. Check if two strings are anagrams using Streams.
  8. Remove duplicates from a list using Java 8.
  9. Find the maximum and minimum element in a list.
  10. Flatten a list of lists using flatMap.
  11. Convert List<Integer> to comma-separated String using Collectors.
  12. Find common elements between two lists using Streams.
  13. Partition numbers into even/odd using Collectors.partitioningBy.
  14. Group employees by department using Collectors.groupingBy.
  15. Find the average salary of employees using Streams.
  16. Find the longest string in a list using reduce().
  17. Merge multiple lists into one using Streams.
  18. Find the top N elements in a list using Streams.
  19. Convert a list into a Map<Key, Value> using Collectors.toMap().
  20. Check if a list contains only unique elements using Streams.
13. Optional Usage (Java 8+)
  1. Safely retrieve a nested object’s field using Optional.
  2. Return a default value if null using orElse() / orElseGet().
  3. Throw an exception if value not present using orElseThrow().
  4. Difference between map() and flatMap() in Optional.
  5. Best practices when using Optional in APIs.
14. Concurrency Coding Challenges
  1. Implement producer-consumer using BlockingQueue.
  2. Print odd/even numbers alternately using two threads.
  3. Count number of words in a file using parallelStream().
  4. Asynchronous computation using CompletableFuture.
  5. Chain multiple futures with thenApply() / thenCompose().
  6. Implement a thread-safe Singleton (Bill Pugh, Double Checked Locking).
  7. Implement a barrier for 3 threads using CyclicBarrier.
  8. Write a program where 3 threads print A, B, C in sequence repeatedly.
15. Collections Coding Challenges
  1. Implement an LRU Cache using LinkedHashMap.
  2. Custom sort an Employee list by salary and name.
  3. Find frequency of characters in a string using HashMap & Streams.
  4. Implement a custom comparator with multiple fields.
  5. Design a bounded blocking queue from scratch.
  6. Implement a multithreaded word counter using ConcurrentHashMap.
16. Tricky & Scenario-Based Questions
  1. Why must HashMap keys be immutable?
  2. Why is String used as key in HashMap?
  3. What happens if hashCode() is not overridden properly?
  4. Difference between ConcurrentModificationException and IllegalStateException.
  5. How to handle large datasets efficiently with Streams?
  6. When should we not use parallel streams?
  7. Can a Stream run infinitely? (Use of Stream.generate() and Stream.iterate()).
  8. Why prefer CompletableFuture over Thread or Executor?
  9. What are the pitfalls of using Optional as a field in entities?
  10. How to debug thread deadlock in a production JVM?
  11. What are some real-world cases where CopyOnWriteArrayList is preferred?
  12. When to prefer LinkedList over ArrayList in performance-critical apps?
  13. What is the impact of load factor tuning in HashMap?
🏁 Conclusion

πŸ”₯ This compilation serves as a professional, structured, and comprehensive 200+ question Java 8+ interview preparation guide, ensuring complete coverage of critical areas:

This guide is crafted as a master reference for interview preparation across startups, product-based firms, and enterprise companies. By revising these questions, practicing coding problems, and mastering practical concurrency and collections scenarios, you will be equipped to excel in both conceptual and applied aspects of Java interviews with confidence.