Essential Java Interview Questions Every Backend Engineer Must Master

The article debunks the myth that most job seekers land offers at big tech firms, highlighting that the majority end up in numerous small and medium companies, and presents a curated list of interview questions that reflect the core requirements of typical Java positions.

Written Test

Java Written Questions (High‑frequency short answer)

What is the difference between Cookie and Session?

What is the difference between a process and a thread?

What is the difference between a Filter and an Interceptor?

What is the difference between HashMap and ConcurrentHashMap, and how does the latter ensure thread safety?

What is a database transaction and what are its four ACID properties?

What are the differences among String, StringBuilder, and StringBuffer?

(Programming) Implement multithreaded alternating output of 0 and 1.

SQL Programming Questions

Typical "student‑teacher‑score" model scenarios.

Count students whose average score exceeds a given threshold X.

Query the ID and name of a student for a specific teacher’s course.

Explain the differences and performance implications of COUNT(*), COUNT(1), and COUNT(column).

Java Interview Topics

Tip: The resume is the target of interviewers' questions; for unfamiliar areas such as the JVM, you may omit them to reduce risk.

Java Fundamentals

What are the three OOP principles (encapsulation, inheritance, polymorphism)?

What is the purpose of the final keyword when applied to classes, methods, or variables?

What impressive new features does Java 17 introduce?

(Hot) What are virtual threads and how do they differ from platform threads?

What is the difference between == and equals()?

What are the differences between abstract classes and interfaces?

Java Collections

(Must) HashMap: underlying data structure (array + linked list/red‑black tree), put process, hash collisions and resolution methods.

Concept of ConcurrentHashMap and implementation differences between JDK 1.7 and 1.8.

Other common collections and their use cases (ArrayList, LinkedList, HashSet, etc.).

Java Concurrency (JUC)

(Must) Thread pool: core parameters (corePoolSize, maximumPoolSize, workQueue), how to configure them for I/O‑bound vs CPU‑bound workloads, working principle, and familiarity with the Fork/Join framework.

Locks: which locks have you used (synchronized, ReentrantLock), typical scenarios, and underlying AQS implementation.

Concurrent utilities: experience with CountDownLatch, Semaphore, CompletableFuture; why use CompletableFuture, problems it solves, internal structure; why use CountDownLatch, problems it solves.

JVM

What is the JVM memory layout (runtime data areas)?

Garbage collection: common marking algorithms (reference counting, reachability analysis) and popular collectors (G1, ZGC) and their selection criteria.

What is the Java class‑loading process and the parent‑delegation model?

Is a Java object always allocated on the heap? Discuss stack allocation and TLAB.

Difference between OOM and memory leak; how to diagnose and locate them.

(High frequency) Do you have JVM tuning experience? How to troubleshoot high CPU, memory saturation, frequent GC, and which commands/tools you use.

Database (MySQL)

Indexes: why they are needed, how they accelerate queries, common scenarios where indexes become ineffective, why B+Tree is used, and reasons for slow queries despite using an index.

Transactions: four isolation levels, their implementation principles, and what MVCC is.

Locks: types of locks in MySQL (row lock, table lock, gap lock, etc.) and how to investigate and resolve deadlocks.

SQL optimization: share your optimization experience, how to read an EXPLAIN plan, and key parameters to watch.

Cache (Redis)

Basic data types, underlying data structures, and typical use cases.

Two persistence mechanisms (RDB, AOF): differences and selection considerations.

Memory eviction policies; differences between LRU and LFU.

(High frequency) What are cache penetration, breakdown, and avalanche, and how to mitigate them?

Framework (Spring)

(Must) Scenarios where Spring transactions fail (non‑public methods, internal calls) and the underlying dynamic proxy principle.

What are Spring’s transaction propagation mechanisms?

(Must) How does Spring Boot’s auto‑configuration work?

How does Spring resolve circular bean dependencies (three‑level cache)?

What is AOP and where have you applied it in projects?

Microservices

Core components of Spring Cloud and their responsibilities.

How does Nacos implement dynamic routing configuration?

When service A calls service B, what is the underlying communication principle (e.g., Feign)?

Message Queue (MQ)

What problems does introducing MQ solve in projects (asynchrony, decoupling, traffic shaping)?

(Reliability triple question) How to guarantee no message loss, avoid duplicate consumption, and handle message backlog?

Project Interview

Core question: interviewers usually ask you to present the project you are most familiar with or that posed the greatest difficulty.

Answer strategy (STAR method):

Situation: Clearly describe project background and business requirements.

Task/Target: Identify technical challenges (high concurrency, data consistency, performance bottlenecks, etc.).

Action: Detail research, solution comparison, final choice, and implementation specifics.

Result: Quantify benefits (QPS increase, latency reduction, productivity gains, etc.).

Tips: Prepare 1‑2 “project highlight” stories in advance and steer the interview toward your strengths; logical consistency is essential.

Interview Review Summary

Recording each interview helps uncover issues you might not notice during the session.

Transcribing recordings into text with tools like Feishu or DingTalk enables quick pinpointing and analysis of weak answers.

Leverage AI to analyze interviewers' questions, identify assessment points, and suggest answer improvements.

Maintain independent thinking; AI is a tool, and final answers should reflect your own experience and judgment.