Java Interview Prep

A hands-on repo to practise Java for interviews. Every topic has a guide, links, exercises with tests, a reference solution, and "twist" ideas to test your own understanding.

Quick start

Requirements: JDK 21 and (optionally) Maven. The Maven Wrapper (./mvnw) is included, so only a JDK is strictly needed.

./run.sh --list              # list categories
./run.sh                     # run everything (unsolved exercises fail, that's the point)
./run.sh collections         # run one category (lists every test with PASS/FAIL/SKIP, time and purpose)
./run.sh core PointTest      # run one test class
./run.sh --solutions         # run against reference solutions (sanity check)
./run.sh --learn core        # solutions + every test with its purpose (read-only study)
./run.sh --details           # per-test list even for a full run
./run.sh --verbose core      # also show the raw Maven output
mvn test -Dtest=PointTest#toStringFormat    # one test method

IDE: open pom.xml in IntelliJ / VS Code; every test has a run/debug button.

How it works

exercises/java/prep/<category>/   <- YOUR CODE: stubs that throw UnsupportedOperationException("TODO")
solutions/java/prep/<category>/   <- reference solutions (same class names; peek after you try)
src/test/java/prep/<category>/    <- tests (same tests verify both)
docs/<category>.md                <- guide: concepts, common questions, gotchas, links

0. Just want to read? Run ./run.sh --learn and follow docs/solutions-tour.md.
1. Read docs/<category>.md.
2. Open an exercise in exercises/, read the Javadoc, implement it.
3. ./run.sh <category> until green.
4. Compare with solutions/. Every method there has a Javadoc explaining what it does, how it works step by step, an example and its complexity. Read the // TWIST: comments too.

-Dsolutions just swaps the compiled source folder, so the same tests run against the reference code.

Test your own learning (twist the code)

• Break it on purpose: follow the // TWIST: comments in solutions/. Predict what fails, run, compare. (e.g. delete hashCode() from Point.)
• Predict the output: src/test/java/prep/core/PuzzlesTest.java and jvm/JvmDemosTest.java. Cover the assertion, guess, then check. Change an expected value to see the failure message.
• Raise the level: tests marked @Disabled("Level 2 ...") are harder variants. Remove @Disabled after you extend the code.
• Add your own edge cases under // YOUR EDGE CASES HERE in each test (null, empty, huge, duplicates, negatives).
• Explain it back: write time/space complexity and your reasoning in a comment above each solution, then compare with the guide.
• Use git as a safety net: git switch -c practice/<topic>; git diff shows your twist, git restore . resets it.

Categories and progress

Tick them off as you go.

Done	Category	Guide	Exercises
[ ]	Core Java	docs/core.md	Point, StringPractice, puzzles
[ ]	Collections	docs/collections.md	LruCache, CollectionPuzzles
[ ]	Modern Java 8-21	docs/modern.md	StreamPractice, ShapeMath
[ ]	Concurrency	docs/concurrency.md	SafeCounter, BoundedBuffer, ParallelSum
[ ]	Algorithms	docs/algorithms.md	Searching, Sorting, ArrayTricks
[ ]	Data structures	docs/datastructures.md	LinkedListOps, MinStack, TreeOps, GraphOps
[ ]	Design patterns	docs/patterns.md	AppConfig, Pizza, EventBus
[ ]	Low-level design	docs/design.md	RateLimiter, ParkingLot
[ ]	JVM and memory	docs/jvm.md	Team, demos
[ ]	Multiple-choice practice	docs/mcq.md	50 questions (core Java + Spring) with the answer shown and runnable demos for every option

Suggested order: core -> collections -> algorithms -> data structures -> modern -> concurrency -> patterns -> design -> jvm.

A real Spring Boot server to play with

./serve.sh starts a small REST API (Spring Boot + JPA + H2) in spring-demo/; call it with curl, watch the request flow and SQL in the console, change the code and see what breaks. Guide with experiments: docs/spring-server.md.

Adding your own exercise

1. Add a stub in exercises/java/prep/<category>/Foo.java and the working version in solutions/java/prep/<category>/Foo.java (same package and class name, same public API).
2. Add src/test/java/prep/<category>/FooTest.java.
3. Check both modes: ./run.sh --solutions <category> must be green; ./run.sh <category> should fail until solved.

Notes

• Links in the guides point at stable, well-known sites (Oracle docs, JEPs, LeetCode, Refactoring Guru, etc.). They were written without network verification, so report any that moved.
• Concurrency tests use timeouts and repeated runs; run them a few times after you think you're done.