Comprehensive Java Interview Topics and Core Concepts

Fundamentals 1) Collections: List vs Set, subclasses (ArrayList, Vector, LinkedList; HashSet, TreeSet). 2) Underlying implementation of HashMap and ConcurrentHashMap. 3) LinkedHashMap for ordered storage. 4) Differences between Hashtable and ConcurrentHashMap. 5) String, StringBuffer, StringBuilder distinctions. 6) Object methods such as wait(). 7) wait vs sleep. 8) JVM memory structure and algorithms. 9) Strong, soft, weak reference differences. 10) Array memory allocation. 11) Design patterns (excluding Singleton) with examples. 12) Core of Spring MVC, request flow, IoC. 13) AOP principle in Spring. 14) MyBatis result set handling via reflection. 15) Java polymorphism examples. 16) Purpose of interfaces. 17) HTTP/HTTPS protocols. 18) TCP/IP protocol suite. 19) OSI seven-layer model. 20) TCP vs UDP. 21) Symmetric and asymmetric encryption algorithms. 22) TCP three-way handshake and four-way termination. 23) Cookie vs session and distributed state management. 24) Git vs SVN. 25) Sample code for stack overflow and heap overflow. 26) Use of ThreadLocal for data sharing.

IO 1) Differences among BIO, NIO, AIO. 2) NIO framework and Dubbo implementation. 3) JD.com's JSF communication protocol (Dubbo protocol).

Algorithms 1) Stack vs heap data structures and rationale. 2) TreeMap insertion using rotations (left, right, double). 3) Inserting into a sorted array using binary search and its time complexity. 4) Time complexity of balanced binary trees. 5) When to use hash algorithms vs tree algorithms. 6) BFS and DFS in graphs, referencing JVM GC implementations.

Multithreading 1) Blocking queue implementations (e.g., ArrayBlockingQueue). 2) Inter‑process communication methods: message queues, shared memory, semaphores, sockets. 3) Common classes from java.util.concurrent. 4) Typical use cases of multithreading. 5) Types of thread pools provided by Executors. 6) Reasons for using thread pools. 7) volatile keyword for variable visibility across threads.

Database (MySQL) 1) MySQL optimization experience. 2) SQL statement tuning tools. 3) Index types: B+ tree, hash, and their appropriate scenarios. 4) Storage engines and their differences. 5) Transaction properties and isolation levels. 6) Pessimistic vs optimistic locking and implementation.

Message Queue (MQ) 1) General principles of MQ systems. 2) Ensuring real‑time delivery. 3) Persistence mechanisms in MQ.

NoSQL (Redis) 1) Differences between Redis and Memcached. 2) Practical uses of Redis. 3) Persistence methods: RDB and AOF. 4) Redis cluster synchronization. 5) Data addition process using hash slots. 6) Eviction policies. 7) Available data structures.

Zookeeper 1) What Zookeeper is. 2) Use cases of Zookeeper. 3) Leader election process. 4) Inter‑cluster communication. 5) Node encryption methods. 6) Implementation of distributed locks.

Linux 1) Common Linux commands. 2) Obtaining the PID of a Java process. 3) Finding the network port of a process. 4) Real‑time log printing. 5) Counting lines containing a specific string.

Design & Thought 1) Experience with code refactoring. 2) Real‑time ranking for ten‑million users. 3) Handling 50,000 concurrent ticket‑booking requests.