How to use this guide
Do not try to master everything. For each subject below you get: why it matters, the focus topics (the ones that actually get asked), and a few sample questions with short answers so you know the depth expected. Read actively: after each topic, try to explain it out loud in one or two sentences. If you can teach it, you know it.
Operating Systems (OS)
Focus on these topics:
Q: Difference between a process and a thread?
A process is an independent running program with its own memory; a thread is a lighter unit inside a process that shares that memory. Threads are cheaper to create and communicate, but a crash in one can affect others in the same process.
Q: What is a deadlock and how do you prevent it?
A deadlock is when processes wait on each other forever. It needs four conditions at once (mutual exclusion, hold-and-wait, no preemption, circular wait). Break any one, for example by acquiring locks in a fixed order, and you prevent it.
📚 Want the full theory with analogies and quizzes? See the Operating Systems Deep Dive.
Databases (DBMS)
Focus on these topics:
Q: What is normalization and why do it?
Normalization organizes tables to reduce duplicate data and avoid update anomalies. You split data into related tables (up to 3NF for most apps) so each fact lives in exactly one place, keeping data consistent.
Q: Explain ACID.
Atomicity (all or nothing), Consistency (valid state to valid state), Isolation (transactions do not corrupt each other), Durability (once committed, it stays). Together they keep transactions reliable, like a bank transfer never losing money midway.
Want hands-on SQL practice? See the SQL & Databases guide. For the full theory with quizzes, see the DBMS Deep Dive.
Computer Networks (CN)
Focus on these topics:
Q: TCP vs UDP?
TCP is reliable and ordered, it confirms delivery (used for web pages, email). UDP is fast but does not guarantee delivery or order (used for live video, games, calls) where speed beats perfection.
Q: What happens when you type a URL and press enter?
The browser resolves the domain to an IP via DNS, opens a TCP connection (and a TLS handshake for HTTPS), sends an HTTP request, the server responds with HTML, and the browser renders it, fetching CSS, JS, and images as needed.
📚 Full theory with quizzes: the Computer Networks Deep Dive. See it live with the Networking Toolkit.
Object-Oriented Programming (OOP)
Focus on these topics:
Q: What are the four pillars of OOP?
Encapsulation (bundle data with the methods that use it, hide internals), Abstraction (expose what, hide how), Inheritance (reuse via parent-child classes), and Polymorphism (one interface, many behaviors, e.g. overriding a method).
Q: Overloading vs overriding?
Overloading = same method name, different parameters, in the same class (compile-time). Overriding = a subclass redefines a parent method with the same signature (run-time). Overload for convenience, override for polymorphism.
📚 Full theory with quizzes: the OOP Deep Dive.
What to prioritize
Short on time? Here is a rough priority order based on how often each shows up in fresher interviews.
| Subject | Interview frequency | Must-know topics |
|---|---|---|
| OOP | Very high | 4 pillars, overloading vs overriding, abstract vs interface |
| DBMS | Very high | Joins, normalization, ACID, indexing |
| OS | High | Process vs thread, deadlock, scheduling, synchronization |
| Networks | Medium to high | TCP vs UDP, HTTP, DNS, URL walkthrough |
Free places to learn
- GeeksforGeeks - topic-wise notes and interview questions for all four subjects.
- Gate Smashers (YouTube) - clear, short lectures on OS, DBMS, and CN.
- roadmap.sh/computer-science - a visual map of the whole CS core.
- cs.fyi - concise guides on key CS concepts.
What to do next
Pair these fundamentals with problem-solving and interview practice:
Many freshers grind DSA but freeze when asked "what is a deadlock?" or "explain normalization." These core subjects come up in almost every entry-level interview, especially at service companies and product firms alike. This guide gives you the high-value topics per subject, plus the questions that actually get asked. Want a topic explained your way? Tap ✦ Ask AI.