00 / Java
Learn debugging, testing and real infrastructuregraded by the real tools
Debug real stack traces, tune real queries, ship 180 lessons deep - graded by the JVM's own tools, not a quiz.
Early access
01 / what grades you
Grading runs the real tools the job actually uses, in the copy of VS Code you already have open - not a simulated sandbox and not a recording.
When a lesson says PASSED, that verdict came from the real debugger, the real test runner, or a real service running on your machine.
02 / the free tier
22 free lessons, forever
No account. No card.. the free tier makes zero network calls, answers ship encoded and are released one failed attempt at a time, and your progress is a local file you can read and delete. no accounts, no telemetry, no subscription.
03 / syllabus
Lesson 1 of 22 free. Opens in your own VS Code - free forever.
Store not yet open What to expect on your machine
180 lessons across 8 tracks
Debugging
Your first breakpointFREE/Stepping through codeFREE/Reading the call stackFREE/Reading stack tracesFREE/Hot code replace and drop frameFREE/Conditional breakpointsFREE/Watch expressionsFREE/Exception breakpointsFREE/Debugging a live HTTP request/Which layer dropped the field?/The silently null field/When the service will not start/Debugging scheduled code/Logpoints: diagnose without stopping/Debugging a failing test/A race you can catch every time/Deadlock: stopped without dying/Controlling threads one at a time/Heap histograms: who ate the memory/Stepping into code you do not own/Remote attach: debugging inside Docker/Two JVMs, one debugger/The mutating quantity/Timeouts and retries/Contract mismatch: the renamed field/One request id, two logs/Breakpoint the consumer/The lost message/The vanishing money/The read that lies/The three-service chain/Reading the trace/The Heisenbug/The lying service/Capstone
Java
Variables and typesFREE/Methods and parametersFREE/Control flow/Your first class/Constructors and state/Encapsulation/Composition/Polymorphism/Object roles/Null and Optional/Exceptions/Integer traps/Equality/Generics/Lambdas and streams/Immutability/Records and sealed types/Mappers/Writing JSON/Reading JSON/The round-trip guarantee/Testing collections/Logging with SLF4J/Lombok/Your first thread pool/CompletableFuture/Sharing state safely/The parallel pipeline/Reading and writing files/Calling APIs with HttpClient/Maven literacy/Virtual threads
Git
Repo anatomyFREE/The three areasFREE/Commits done well/Ignoring the right things/Branching/Two kinds of merge/The conflict/Branch lifecycle/Amend/Rebase onto a moved main/Squash and fixup/Reword, reorder, drop/Cherry-pick and revert/Reflog, the undo of undos/Reset: soft, mixed, hard/Stash/Bisect/Remotes: clone, fetch, pull, push/Pull with rebase/Force with lease, and tags/The feature workflow/Capstone: inherit the mess/What a commit really is/Trees, blobs and refs/The index unmasked/Submodules without tears/Worktrees/Large files and history bloat/Attributes that save releases/Hooks that guard the gate/Signed work/Rebase with a robot/Autosquash discipline/The reverted merge trap/Detached HEAD, no fear/Archaeology/Grand capstone: the inherited repo
Testing
Your first JUnit testFREE/Assertions that read wellFREE/Fixtures and lifecycle/Parameterized tests/Testing exceptions/Black-box vs white-box/Test doubles: stubs vs mocks/Designing for mocks/Verify and captors/TDD capstone
SQL
The server and your first queryFREE/Tables and typesFREE/First inserts/SELECT and WHERE/Order, limit, distinct/Aggregates/HAVING/CASE and conditional aggregation/INNER JOIN/LEFT JOIN and the NULL side/The fan-out trap/Self-join/Anti-joins and the NOT IN landmine/UPDATE, DELETE, RETURNING/Constraints/UPSERT/NULL three-valued logic/Transactions/SQL injection/Subqueries and CTEs/Set operations/Dates and strings/Window functions/Views/jsonb/Indexes and EXPLAIN/Fix the slow query/Recursive CTEs/Schema design/Capstone: the wrong report
Spring Boot
Beans and injectionFREE/Your first controllerFREE/Request data/The service layer/Configuration and profiles/Validation/JPA repositories/Transactions done right/Mappers at the boundary/MockMvc blackbox/Slice tests and mocked beans/Integration tests/Problem responses/Security basics/Capstone: build the API
Spring Advanced
The JWT resource serverFREE/Method securityFREE/The RabbitMQ listener/Retries and the dead letter queue/Kafka basics/Consumer groups/@Cacheable on Redis/Cache TTL and the stampede/A Temporal workflow/The stuck workflow
Infrastructure
Docker imagesFREE/Docker composeFREE/Debugging in containers/Redis basics/Redis as a cache/Kubernetes deploy/Debug a pod/Config and secrets/Multi-stage builds/Volumes and persistence/Crash forensics
04 / how it grades you
Do the work
Open the lesson in VS Code and write the fix, the query, or the test - in your own editor, with your own tools.
The real tool checks it
The debugger, the test runner, or the real service your code talks to decides pass or fail - not a string match on your keystrokes.
Hints, then the solution
Stuck lessons escalate through hints before the reference solution unlocks - spoilers are earned, not browsable.
05 / your machine
What to expect on your machine
By track, per operating system
| Track | Windows | macOS | Linux |
|---|---|---|---|
| All 180 lessons | 177/180 3 skip | 180/180 | 180/180 |
transcribed from the product's delivery board (run 30398707786), 2026-08-29
curriculum lessons; the one-lesson First Run track is not counted on this site
CI-proven Windows
the full 180-lesson gate is CI-green on all three operating systems - Windows, macOS and Linux - with real Postgres, Redis and RabbitMQ provisioned natively on every leg, and the encrypted paid edition walked as a buyer on all three. The store is not yet open.
CI-proven, hand pass pending macOS
macOS is CI-proven on Apple Silicon (the Intel path is the identical JVM path, not yet run on real Apple hardware), and it goes on sale the moment it passes a hand pass there, not a day sooner.
CI-proven Linux / WSL
the full 180-lesson gate is CI-green on all three operating systems - Windows, macOS and Linux - with real Postgres, Redis and RabbitMQ provisioned natively on every leg, and the encrypted paid edition walked as a buyer on all three.
Store not yet open 22 free lessons are ready right now.
06 / receipts
Receipts
Receipts are claims paired with evidence. The changelog carries the dated record.
PASS the paid edition is encrypted, and the whole encrypted path is CI-proventhe full curriculum ships as an encrypted pack that only unlocks under your licence; CI walks every paid lesson the way a buyer would - decrypt, grade, done - on Windows, macOS and Linux, all 158, so it is ready the day the store opens
PASS 35 debugging lessons, breakpoint to distributedraces engineered to reproduce every run, deadlocks you observe mid-lock, heap forensics, remote attach into Docker, a three-service chain, a Heisenbug, a capstone
PASS 172 lessons carry a CI solvability proofthe planted bug must fail and the reference solution must pass before a lesson ships; the other 8 are documented terminal walkthroughs
PASS interactive rebase happens in your editorthe Git track sets sequence.editor to VS Code - the todo file opens where you work, with a reflog rescue lesson taught first
PASS the SQL capstone's bugs are independently layeredfix the join fan-out alone and the report is still wrong; fix the NULL trap alone, still wrong; the numbers only agree when you find both
PASS Java installs itselfGradle auto-provisions JDK 21 on first build - clone to first green checkmark without touching a JDK download page
PASS guided first run from a cold installa Start-here Lesson 0, an activation check that names any missing tool, a VS Code walkthrough and a one-click Prepare sandbox button
PASS zero minutes of videonothing to watch; everything to do
07 / questions
Do I need an account?
No. The extension is local. Your progress lives on your machine and exports to a file when you move machines.
What do I need installed?
VS Code, git, and Docker (for the SQL and infrastructure tracks). Java installs itself on first build. A doctor script checks your machine in one command.
I use AI to code. Is this for me?
Especially you. Use the AI - ship faster. But when the stack trace arrives, autocomplete goes quiet. These are the skills that answer.
Can I see the answers?
Each lesson coaches you with escalating hints before revealing a reference solution. Spoilers are earned, not browsed.
Refunds?
14 days, no questions.