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316+ questions across 13 categories. Topics are grouped into tracks and ordered so each builds on the last — pick a topic, or search to jump straight to an answer.

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Java & the JVM

Language fundamentals and collections first, then modern Java, concurrency, and what happens inside the JVM.

Start withWhat is a HashMap?

  1. Core Java

    29 questions    OOP, Collections, Strings, Exceptions, JVM internals.8 easy · 15 medium · 6 hard

  2. Java Collections

    23 questions    List/Set/Map internals, HashMap, ConcurrentHashMap, performance.4 easy · 13 medium · 6 hard

  3. Java 8+

    10 questions    Lambdas, Streams, Optional, functional interfaces.2 easy · 8 medium

  4. Multithreading

    30 questions    Threads, locks, executors, the memory model, deadlocks, async.5 easy · 12 medium · 13 hard

  5. JVM

    26 questions    Memory areas, garbage collection, JIT, class loading, tuning.4 easy · 13 medium · 9 hard

  6. Advanced Java

    28 questions    Concurrency, JVM tuning, memory model, virtual threads.12 medium · 16 hard

APIs & distributed systems

How services talk to each other: HTTP semantics and payloads, then service boundaries, and finally designing for scale.

Start withExplain the main HTTP status code families.

  1. REST APIs

    24 questions    HTTP verbs, status codes, idempotency, versioning.6 easy · 12 medium · 6 hard

  2. JSON

    26 questions    50 curated JSON interview questions — syntax, parsing, schema validation, and JSON vs XML, with worked answers for every experience level.15 easy · 10 medium · 1 hard

  3. Microservices

    20 questions    Service design, communication, resilience, sagas, events, observability.4 easy · 10 medium · 6 hard

  4. System Design

    21 questions    Scaling, caching, load balancing, CAP, messaging.4 easy · 10 medium · 7 hard

Data & cloud

Relational data and query performance, then the core AWS building blocks.

Start withExplain INNER JOIN vs LEFT JOIN.

  1. SQL

    22 questions    Joins, indexes, transactions, query optimization.5 easy · 11 medium · 6 hard

  2. AWS

    22 questions    EC2, S3, Lambda, IAM, VPC and the well-architected pillars.5 easy · 11 medium · 6 hard

Python

Core data types and idioms, then functions, OOP, generators, and the GIL.

Start withWhat is the difference between a list and a tuple in Python?

  1. Python

    27 questions    Data structures, comprehensions, decorators, the GIL.16 easy · 10 medium · 1 hard