100 imp interview questions for Java developers

Core Java

1. What is the difference between == and .equals() in Java?
2. Explain the concept of immutability in Java.
3. What is the purpose of the final keyword in Java?
4. How does Java achieve platform independence?
5. What is the difference between String, StringBuilder, and StringBuffer?
6. Explain the concept of method overloading and overriding.
7. What is the use of the super keyword in Java?
8. How do you handle exceptions in Java? Explain try-catch-finally.
9. What is the difference between checked and unchecked exceptions?
10. What is the purpose of the static keyword in Java?
11. Can you overload the main method in Java? If yes, how?
12. What is the difference between ArrayList and LinkedList?
13. How does Java garbage collection work?
14. What is the difference between HashMap and Hashtable?
15. Explain the concept of multithreading in Java.
16. What are the different ways to create threads in Java?
17. What is the difference between sleep() and wait()?
18. What is the purpose of the volatile keyword?
19. How does Java support encapsulation?
20. What is the difference between an abstract class and an interface?
21. Can we instantiate an abstract class? Why or why not?
22. What is the difference between compareTo() and compare()?
23. What is autoboxing and unboxing in Java?
24. What is the difference between transient and volatile keywords?
25. What is the purpose of the Object class in Java?

Collections Framework

1. What is the difference between List, Set, and Map?
2. How does HashSet ensure uniqueness of elements?
3. What is the time complexity of operations in ArrayList vs LinkedList?
4. What is the difference between Iterator and ListIterator?
5. How does TreeSet maintain sorting order?
6. What happens if we add a null value to a HashMap?
7. What is the difference between poll() and remove() in Queue?
8. Explain the working of ConcurrentHashMap.
9. What is the difference between fail-fast and fail-safe iterators?
10. How does PriorityQueue work?
11. What is the difference between Comparable and Comparator?
12. What is the purpose of the retainAll() method in collections?
13. How does Vector differ from ArrayList?
14. What is the difference between keySet() and entrySet() in Map?
15. How do you sort a List of custom objects?

Java 8 Features

1. What are lambda expressions in Java?
2. What is a functional interface? Give examples.
3. What is the difference between forEach and for-loop?
4. Explain the concept of streams in Java.
5. What is the difference between map() and flatMap()?
6. What are default methods in interfaces?
7. What is the purpose of the Optional class?
8. How does Stream.collect() work?
9. What are method references in Java?
10. What is the difference between intermediate and terminal operations in streams?

OOPs Concepts

1. What is polymorphism? Explain its types.
2. What is inheritance? What are its advantages and disadvantages?
3. What is the difference between composition and aggregation?
4. What is the diamond problem in Java? How is it resolved?
5. What is the difference between this and super?
6. What is the difference between abstract classes and interfaces in Java 8+?
7. What is encapsulation? Why is it important?
8. What is the difference between private, protected, and public access modifiers?
9. What is the purpose of the toString() method?
10. What is the difference between instanceof and isInstance()?

Advanced Java

1. What is the difference between JVM, JRE, and JDK?
2. What is reflection in Java? Provide an example.
3. What is the purpose of annotations in Java?
4. What is the difference between ClassNotFoundException and NoClassDefFoundError?
5. What is the difference between String pool and heap memory?
6. What is the purpose of the ClassLoader in Java?
7. What is the difference between serializable and externalizable?
8. What is the purpose of the clone() method?
9. What is the difference between shallow copy and deep copy?
10. What is the purpose of the finalize() method?

Concurrency and Multithreading

1. What is the difference between Runnable and Callable?
2. What is the purpose of the ExecutorService?
3. What is thread pooling? Why is it used?
4. What is the difference between synchronized and ReentrantLock?
5. What is the difference between start() and run() methods in threads?
6. What is thread starvation? How can it be avoided?
7. What is a deadlock? How can it be prevented?
8. What is the purpose of ThreadLocal?
9. What is the difference between notify() and notifyAll()?
10. What is the purpose of the ForkJoinPool?

Spring Framework (if applicable)

1. What is dependency injection? How does Spring implement it?
2. What is the difference between @Autowired and @Qualifier?
3. What is the difference between @Component, @Service, and @Repository?
4. What is the purpose of the @Transactional annotation?
5. What is Spring Boot? How is it different from Spring Framework?
6. What is the purpose of the application.properties file?
7. What is the difference between singleton and prototype scopes in Spring?
8. What is AOP (Aspect-Oriented Programming)?
9. What is the difference between @Controller and @RestController?
10. What is the purpose of the DispatcherServlet?

Hibernate (if applicable)

1. What is the difference between get() and load() in Hibernate?
2. What is lazy loading? How does it work?
3. What is the purpose of the SessionFactory in Hibernate?
4. What are the different states of an entity in Hibernate?
5. What is the difference between save() and persist()?
6. What is the purpose of the @Entity annotation?
7. What is HQL (Hibernate Query Language)?
8. What is the difference between merge() and update()?
9. What is the second-level cache in Hibernate?
10. What is the purpose of the @Transactional annotation in Hibernate?