What Core Java topics are asked in backend interviews?

Panels usually probe JVM basics (JDK vs JRE vs JVM, class loading), OOP and object contracts (equals/hashCode, immutability), collections and HashMap internals, exception design, generics and PECS, Java 8+ (lambdas, streams, Optional, java.time), multithreading and the Java Memory Model, garbage collection, I/O and serialization, and newer releases (records, sealed classes, virtual threads, JPMS). This path covers all of them in order with interview-style lessons and quizzes.

How should a fresher prepare for a Java interview?

Start with platform fundamentals and OOP (sections S01–S02), then object contracts and exceptions. Read every L1 (fresher) lesson free in full, pass the 3-question checkpoint after each lesson, and use the linked Java practice bank for spaced repetition. Follow the weekly study plan (~30 minutes per day) or jump to collections and multithreading when your job description stresses them.

Does this course cover Java 8 through Java 25?

Yes. Lessons are tagged with a Java baseline where it matters (e.g. streams in Java 8, text blocks and records in newer LTS releases, virtual threads and structured concurrency in Java 21+). The path hero shows Java 8 → 25 so you know modern language features are included alongside classic interview favourites.

How long does it take to finish Core Java interview prep?

The full path is about 19 hours of reading across 96 lessons. At 30 minutes a day, five days a week, the built-in study plan finishes in roughly ten weeks. You can filter to fresher basics or senior topics if you only have a few days before a screen.

Is Core Java content free on PreparationZone?

Every fresher-level (L1) lesson is free in full, plus all twenty section quick checks. Intermediate through expert lessons (L2–L4) are Pro; each still opens with a free preview so you can judge depth before upgrading.

What is the difference between JVM, JRE, and JDK?

The JDK (Java Development Kit) includes development tools such as javac. The JRE historically shipped the runtime libraries plus a JVM; modern JDK distributions bundle the runtime. The JVM (Java Virtual Machine) executes bytecode on a specific operating system and CPU. Interviewers expect you to say programs are portable via bytecode while the JVM is platform-dependent — covered in section S01.

Blocking I/O vs NIO

Name: Core Java Interview Questions & Prep Path (20 Sections, 96 Lessons)
Availability: InStock
Author: Kishore Bhosale

Channels, selectors and when non-blocking still bites — the danger topics interviewers probe.

IntermediatePro12 min readBy Kishore BhosaleReviewed 2026-10-23

Read these first

Reading and Writing Data

Aspect	Blocking I/O (java.io)	NIO (java.nio)
Model	Thread waits on read/write	Channel + Buffer; selector multiplexes
Strength	Simple code, JDBC-friendly	Many idle connections, one thread
Risk	Thread explosion at scale	Complex state machines, partial reads

Model
Blocking I/O (java.io)Thread waits on read/write
NIO (java.nio)Channel + Buffer; selector multiplexes
Strength
Blocking I/O (java.io)Simple code, JDBC-friendly
NIO (java.nio)Many idle connections, one thread
Risk
Blocking I/O (java.io)Thread explosion at scale
NIO (java.nio)Complex state machines, partial reads

Interview trap

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Test yourself

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