Topic You Must Revise Before Your Java Interview

When I start preparing for company switch. I was confused should I prepare from the basics or should I more focus on the important topics. And based on my last 6 years of experience I can definitely suggest that, No need to start from basics. Focus more on the important topics. And below I have categorized important topics that you must prepare before your next interview.

Core Java Topics (You should have a solid grasp on these):

  1. OOP Concepts:
    • Inheritance, Polymorphism, Encapsulation, Abstraction.
    • Example-based questions on inheritance, method overriding/overloading, encapsulation, and abstract classes/interfaces.
  2. Exception Handling:
    • Difference between checked and unchecked exceptions.
    • try-catch-finally block, throw vs. throws.
    • Custom exceptions: How to create and use them.
    • Handling multiple exceptions, multi-catch and try-with-resources (for AutoCloseable resources).
  3. Collections Framework:
    • List, Set, Map interfaces, and their implementing classes (like ArrayList, HashSet, HashMap).
    • Thread-safe collections like ConcurrentHashMap and CopyOnWriteArrayList.
    • Collections utility class, sorting, filtering, and transformation operations.
  4. Streams and Lambda Expressions (Java 8+):
    • Streams API: filter, map, reduce, collect, etc.
    • Lambda expressions and their use cases.
    • Method references and functional interfaces.
    • Optional class: usage to avoid null checks.
  5. Java Memory Management and Garbage Collection:
    • How Java memory model works (Heap vs Stack).
    • Garbage collection: What is it, different types (Minor GC, Major GC), and how JVM manages memory.
    • Finalization and the finalize() method.
    • Memory leaks: Common causes and how to prevent them.
  6. Concurrency and Multithreading:
    • Thread creation (extends Thread vs implements Runnable).
    • Synchronization and deadlock resolution.
    • Executor framework (ExecutorService, ThreadPoolExecutor).
    • CountDownLatch, CyclicBarrier, Semaphore: How they work and when to use them.
    • Thread safety: Making your classes thread-safe using synchronization or other methods (like Atomic variables).
  7. Design Patterns:
    • Singleton (thread-safe variants), Factory, Builder, Observer, Decorator, Strategy.
    • Understand creational, structural, and behavioral patterns.
    • Real-life scenarios where you’ve applied design patterns (you might get asked for this).
  8. Java I/O:
    • Byte vs Character stream.
    • BufferedReader, FileReader, FileWriter, PrintWriter.
    • Serialization: What it is, how to implement it, and use cases.
    • NIO (New I/O): Channels, Buffers, and non-blocking I/O (Optional but can be useful for a senior role).
  9. Java 8 Features (if you’re not already well-versed, this is crucial):
    • Default methods in interfaces.
    • Streams API (filter, map, reduce).
    • Optional class.
    • Date and Time API (java.time): LocalDate, LocalTime, LocalDateTime, Period, Duration.
  10. Java 9, 10, 11, and beyond (Optional but important for senior roles):
    • Module system (Java 9): Modular applications.
    • Var keyword in Java 10.
    • New HTTP client API (Java 11).
    • New features added in Java 12+, especially if the company is using a recent Java version.

Frameworks & Libraries (Key for senior Java roles):

  1. Spring Framework:
    • Spring Boot: Why use it, how it simplifies development.
    • Spring Dependency Injection: Constructor-based vs Setter-based DI.
    • Spring MVC: Controllers, RequestMapping, RESTful web services.
    • Spring Security: Authentication, authorization, JWT tokens.
    • Spring Data JPA: Repository pattern, Spring Data features.
    • Spring AOP (Aspect-Oriented Programming): Cross-cutting concerns.
    • Spring Boot Actuator: Health checks, metrics, application management.
  2. Hibernate/JPA:
    • Entity mapping, One-to-Many, Many-to-One, Many-to-Many relationships.
    • Lazy vs Eager fetching, FetchType, and performance considerations.
    • HQL and Criteria API for querying.
    • Transactions, Session management, and caching.
  3. RESTful APIs:
    • Creating RESTful services using Spring Boot.
    • HTTP methods (GET, POST, PUT, DELETE) and status codes.
    • Content negotiation (JSON, XML).
    • Swagger for API documentation.
    • Handling request validation and error handling.
  4. Microservices:
    • How to design microservices with Spring Boot and Spring Cloud.
    • Service discovery (Eureka, Consul).
    • Circuit breakers (Hystrix, Resilience4j).
    • API Gateway (Zuul, Spring Cloud Gateway).
    • Inter-service communication (REST, messaging queues like Kafka, RabbitMQ).

Databases:

  1. SQL:
    • Joins (INNER, LEFT, RIGHT, FULL), GROUP BY, HAVING, ORDER BY.
    • Subqueries, Transactions, Indexes, and Normalization.
    • SQL optimization and understanding EXPLAIN plans.
    • Writing efficient queries and understanding their performance.
  2. NoSQL (Optional):
    • Basics of NoSQL databases like MongoDB, Cassandra, and their use cases.
    • Working with JSON in NoSQL systems.

DevOps & Build Tools:

  1. Maven vs Gradle: Dependency management, build lifecycle, and tasks.
  2. Docker: Basics of containerization and how to run Java apps in Docker containers.
  3. CI/CD: Jenkins, GitLab CI, and how to automate build and deployment pipelines.
  4. Version control: Strong knowledge of Git (branches, merge conflicts, rebasing, cherry-pick).

Testing:

  1. Unit Testing:
    • JUnit 5: Writing unit tests with JUnit.
    • Mockito: Mocking objects for unit testing.
    • Test Driven Development (TDD): Writing tests before code.
  2. Integration Testing:
    • Testing with Spring Boot (using @SpringBootTest, @MockBean).
    • Database integration testing with H2 or TestContainers.

System Design (Highly Important for Senior-Level Interviews):

  1. Design scalable and high-performance systems.
  2. Understand load balancing, caching, database sharding, API rate-limiting.
  3. High availability and fault tolerance strategies.
  4. Microservices architecture and designing systems that scale.

Soft Skills and Behavioral Questions:

  1. Leadership: Experience leading teams, mentoring juniors, code reviews.
  2. Problem-solving: How you approach debugging, optimizing code, and handling unexpected issues.
  3. Collaboration: How you work with cross-functional teams (product, design, QA).
  4. Communication: Explaining complex technical topics clearly.

Let me know in the comment, If you feel I have missed anything which is important. And All The Best for your next interview.

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