Master Java Interview Prep: 100+ Essential Backend Questions & Answers

1. Java Related

Design of optimistic and pessimistic locks, ensuring atomicity and the problems they solve.

Byte size of char and double, and their memory layout.

Object memory layout and the object death process.

Object header details.

Deep dive into sync mechanism, exception handling inside sync, deadlock detection and alternatives.

Collections: source code of HashSet and HashMap, and how to make them thread‑safe.

Purpose of multithreading and what thread pools solve.

Thread pool design, parameters, work queue and thread queue structures, and custom design considerations.

AQS principle, ReentrantLock source code and overall design.

Further multithreading source code discussion and a scenario design question.

Memory representation of float vs. double literals.

Source code of String.split and resulting array count for patterns like "a|b|c".

List all JUC (java.util.concurrent) classes, usage, and native thread operations.

Open/closed principle, factory method, builder pattern differences, and hands‑on implementation.

JVM large‑page mode and memory model.

Agile development, defensive programming, parallel programming, and team‑lead perspectives.

Escape analysis purpose and applications.

Criteria for thread‑safety, Java synchronization keywords and design rationale.

Thread coordination example: t1, t2 must finish before t3.

Design and benefits of postfix expressions, with infix counterpart.

Approach to performance tuning, OOM analysis, and memory leak detection.

When and how to analyze thread count, factors to consider.

Difference between abstract and class methods; possibility of static abstract methods.

Java cloning mechanism.

Issues like OOM, JVM optimization, source‑code problems, database tuning, multithreading.

Causes of high CPU usage and mitigation strategies.

Experience with critical sections and their resolution.

Purpose of the volatile keyword.

Implementation of Java polymorphism.

Explanation of spin locks.

Explanation of semaphores.

Triggers for class loading.

Strategies for severe Java memory jitter optimization.

2. Database Related

SQL optimization ideas, relationship between composite indexes and underlying tree structures, and why B+ / B‑ trees are used.

MySQL clustering concepts, encountered problems, and SQL tuning.

Handling massive data volumes: design considerations for 1 million, 10 million, and billions of records.

Comprehensive list and analysis of MySQL configurable parameters beyond max connections.

Optimized index designs and improvement techniques.

Differences between red‑black trees and balanced trees, and why databases prefer other structures.

MySQL lock types, purpose of intention locks.

Optimization strategies for high‑concurrency scenarios.

Conditions under which indexes become ineffective.

3. Data Structures & Operating System

Discussion of learned data structures, especially trees, queues, and B‑trees.

Operating system memory management: segmentation, paging, and virtual memory.

Page replacement algorithms, their varieties, and existence of an optimal algorithm.

Operating system concepts: kernel vs. user space.

Hand‑written reverse linked list.

Quick sort implementation with detailed partition steps and final result.

Bit‑checking technique to determine if the K‑th bit of an integer is 1.

Transform recursive quick sort to non‑recursive and benefits of the approach.

Examples of divide‑and‑conquer algorithms.

4. Networking

Details of HTTP request headers.

Differences between HTTP and HTTPS, versions 1.0/1.1/2.0, long vs. short connections, HTTPS workflow, and public‑key tampering mitigation.

GET vs. POST differences and packet‑capture simulation.

In‑depth explanation of cookies, sessions, tokens, and OAuth 2.0.

Congestion control algorithms and their mechanisms.

Socket fundamentals, read/write buffers, sliding window design rationale.

HTTP Basic Authentication.

HTTPS handshake process.

5. Frameworks

Spring source code overview.

Spring annotations @Autowired, @Resource and their parsing process.

Architecture layers: access layer, service layer, technology stack including Spring Boot, Spring Cloud, Docker.

Advantages of Spring IoC compared to direct object instantiation.

Servlet lifecycle and singleton nature.

Spring MVC initialization flow.

6. Distributed Systems

Varieties of RPC frameworks.

Purpose of consistent hashing.

Design considerations for building high‑concurrency, high‑availability systems.

Cache concepts, problems solved, and backend cache types.

Distributed lock mechanisms.

System cache design rationale and scenarios.

Peak‑shaving techniques using Redis or MQ.

Why message queues help with peak‑shaving and the problems they address.

7. Design Problems

Select top‑10 hot search keywords from billions of logs using only a 2 GB machine.

Algorithm to compress a URL.

Probability‑based ad display algorithm with ratios 1:2:3:4:5 for five ads.

Sorting 25 horses on five tracks with the minimum number of races.

8. Other Topics

Tomcat caching concepts and types.

Lucene principles, inverted index, Chinese tokenization methods.

Large‑scale Top‑N problem.

Access‑layer considerations, encountered issues, and system building factors.

Project performance optimization group responsibilities.

Maven source‑code principles, dependency conflict resolution.

Code quality assessment criteria.