Java-Interview-Questions

This is a list of compiled question asked in recent interviews. This collection of questions should cover a wide range of essential Java topics, from foundational concepts to advanced Spring and microservices topics. Each question serves as a basis for deeper exploration during interviews

1. Platform Independence and JVM

• Explain how Java achieves platform independence.
• What are JVM, JRE, and JDK? How do they differ?

2. Class Loading

• What is a ClassLoader? How is it utilized in Java?
• Explain the difference between ClassNotFoundException and NoClassDefFoundError.
• Why is ClassNotFoundException encountered only at runtime?

3. String Pool and String Handling

• What is the String Pool, and where is it located?
• Describe the intern() function in Java.
• What is the difference between == and equals() for string literals?
• How do == and equals() behave differently for String objects?

4. Object-Oriented Programming

• Define encapsulation in Java.
• What is the Serializable interface, and when is it used?
• Can a Java class be static, final, or private? Explain each.
• Explain Object-Oriented Programming (OOP) and its principles.
• How does OOP differ from scripting languages?

5. Java Exceptions

• What is the superclass of all exceptions?
• Explain the difference between errors and exceptions.
• What are checked and unchecked exceptions?
• What is the try-with-resources block?
• Explain exception propagation in Java.

6. Java Language Features

• What is autoboxing in Java?
• Describe some key Java 8 features.
• What is a lambda function, and how is it useful?
• How does indexing work internally in Java collections?
• Can a constructor be present in an interface? Why not?
• Performance difference between a traditional for loop and an enhanced for-each loop?

7. Threads and Synchronization

• Write code to make 5 threads print data one after the other.
• What is the difference between volatile and atomic operations?

8. Design Patterns

• Explain the Builder Pattern and provide code for it.
• Write code for the Singleton Pattern.
• Describe the Factory Pattern and any issues you’ve encountered with it.
• Why are design patterns needed?
• How do design patterns simplify programming?
• What design patterns are you familiar with?
• How would you invent a new design pattern?

9. Spring Core

• What is Spring Boot, and why is it popular?
• Explain the purpose of annotations like @Qualifier, @Value, @Bean, @Component.
• Difference between @Controller and @RestController.
• Difference between @Mock and @InjectMock.

10. Spring Security

• How does OAuth work?
• Explain how to implement role-based access control using Spring Security.

11. Spring Data JPA

• Name and describe the three main interfaces of Spring Data JPA.

12. Error Handling

• How do you handle exceptions in a controller to return proper status codes?

13. Transaction Management

• What is transaction propagation in Spring Boot?

14. Miscellaneous (Spring)

• What is the application.properties file?
• How does Spring/Hibernate generate SQL queries automatically?
• Describe MVC architecture and the differences between Controller, Service, and Repository layers.
• How is pagination implemented in Spring Boot with an SQL database?
• How do you create unique constraints in a database entity class?

15. Collections API

• How does HashMap work?
• Difference between HashMap and HashTable.
• Compare HashMap and ConcurrentHashMap.
• Differences between List, Set, and Map. When to choose each?
• What is a LinkedList?
• What is the Comparable interface?
• How is hashCode generated for an object?
• How many collection implementations are you aware of?
• What is the Iterable interface?

16. Streams API

• What is the Streams API?
• How many ways are there to create streams in Java?
• Difference between Streams and Collections.
• What are the two types of operations in the Stream API?
• Mention some methods you use in Streams.
• Difference between map() and flatMap().
• What does the collect() method do in Streams?
• Difference between findFirst() and findAny() in Streams?
• How to handle exceptions in Streams?
• How to convert a Stream into a Collection?
• Difference between sequential and parallel streams? Any performance effects?

17. Functional Interfaces

• What is a functional interface?
• Can a functional interface contain default and static methods?
• Provide an example of a functional interface.

18. Microservices vs Monolithic Architecture

• Differences between microservices and monolithic architecture.

19. Distributed Systems

• What is the CAP theorem? Which trade-offs are important for distributed systems?

20. Singleton and Factory Pattern

• Write pseudo-code for a Singleton Pattern.
• Refine code for the Factory Pattern.

21. Immutability

• How do you make a class immutable?

22. Miscellaneous Topics

• Differences between PUT, POST, and PATCH.
• REST API URL naming conventions.
• Explain Maven scopes (e.g., compile, runtime, test).
• What Java versions are you familiar with?
• Key differences between Java versions.

23. Employee Data Processing (Coding Problems)

• Sort a list of employees by highest salary.
• Find employees with salaries greater than a specific value.
• Calculate the average age of employees earning above a certain salary.
• Find the top 3 highest-paid employees.
• Count the number of employees from a specific city.
• Calculate the total salary of a given list of employees.
• Extract the addresses from an employee list.
• Group employees by city.

24. Troubleshooting

• Describe your troubleshooting steps if a backend service is inaccessible after deployment.