The Java reference map
Every topic, concept and question a Java interview draws on, organised so you can see the whole landscape and find what you need. Tracks fill in one at a time; a track appears in navigation once it has written entries.
- Tracks
- 10
- Topics
- 12
- Concepts
- 71
- Questions
- 207
Core Java
The language itself and the standard library. Everything else assumes this, and it is where the majority of interview time is spent regardless of the role.
12 topics · 71 concepts · 207 questions
Language basics
8 concepts · 23 questionsValues, types, operators and control flow. The one idea that matters more than all the syntax is the difference between a primitive and a reference.
Objects and OOP
10 concepts · 30 questions · 2 writtenClasses, the four pillars, and the design judgement behind them. The topic most tutorials reduce to definitions and most interviews probe for reasoning.
Strings
6 concepts · 16 questions · 2 writtenThe most-used class in Java and the one carrying the most out-of-date folklore — PermGen, char[], and "+ becomes StringBuilder" were all true once and are not now.
Collections
8 concepts · 25 questions · 2 writtenThe densest interview territory. Where implementation detail becomes a performance argument you can defend with numbers instead of a Big-O table.
Generics
4 concepts · 12 questionsCompile-time type safety over a runtime that erases it. Almost every confusing generics error traces back to erasure or to variance.
Exceptions
5 concepts · 15 questions · 1 writtenWhere the difference between a ten-minute incident and a two-day one is decided. Handling failure well is a senior skill that starts with not destroying the cause.
Functional Java
6 concepts · 16 questionsThe largest change in how Java is written, and the source of the most confidently wrong interview answers — laziness, parallelism, and checked exceptions in lambdas.
Concurrency
9 concepts · 26 questionsWhere senior rounds are decided. Every question here has a wrong answer that sounds correct, and correctness cannot be established by running it once.
JVM and memory
6 concepts · 19 questionsStop guessing about performance and memory. The goal in this topic is a measurement habit, not a set of numbers to recite.
Modern Java
3 concepts · 8 questionsWhat changed after 8, and what it replaced. Knowing the boundary is what separates experience from having read a recent blog post.
I/O and files
3 concepts · 9 questionsReading and writing the outside world. The two mistakes that never stop happening are ignoring the charset and leaving a stream open.
Dates and time
3 concepts · 8 questionsjava.time replaced two broken APIs. Most remaining bugs are a confusion between an instant, a local time, and a zoned time.
Tracks being added
These are mapped out and filled in one track at a time. Their place in the structure is fixed, so nothing gets restructured as they land.
Spring and Spring Boot
The framework most Java jobs actually use. Dependency injection, auto-configuration, data access, transactions and security — and the questions are about mechanisms, not annotations.
Databases and persistence
SQL, indexing, transactions and isolation, plus JPA and Hibernate — where an ORM's convenience turns into an N+1 query problem nobody noticed.
Microservices and APIs
REST and API design, service communication, resilience patterns, messaging and observability. Mostly questions about failure.
Testing
JUnit, mocking, integration testing with real dependencies, and the judgement about what is worth testing at all.
Design patterns
The classic patterns as they actually appear in the JDK and in Spring, plus the anti-patterns that come from applying them too eagerly.
Data structures and algorithms
What product-company rounds test alongside Java — complexity, the core structures, and the handful of techniques that cover most problems.
System design
Scaling, caching, queues, storage choices and consistency trade-offs, with the case studies that senior rounds are built from.
Build and delivery
Maven and Gradle, dependency management, containers, and the pipeline that gets code to production.
Security
Authentication and authorisation, the OWASP failures that matter for a Java service, and handling secrets and crypto without inventing anything.