Java & Kotlin Handbook.
Java & Kotlin from Zero — a visual, deep-dive handbook that takes you from installing the JDK through the JVM, virtual threads, coroutines, Flow and Kotlin Multiplatform, with equal weight on what Java does well and where Kotlin is simply better.
Two languages, one honest comparison.
Java and Kotlin are usually taught separately, which makes it hard to see why the second one exists. This handbook teaches them together: Part I builds solid Java, Part II moves into Kotlin, and every Kotlin chapter says plainly what changed and why it matters.
Like the Rust handbook, it is a single self-contained page with no build step — 157 callouts, 42 comparison tables and a glossary that covers the vocabulary of both languages.
Three parts, 32 chapters.
- Part I — Java (18 chapters): setup, types, control flow, OOP, records/enums/sealed, generics, collections and streams, I/O, exceptions, lambdas and Optionals, reflection, threads and the memory model, executors and virtual threads, the JVM, builds and tests, performance.
- Part II — Kotlin (10 chapters): why Kotlin exists, null safety, data classes, extensions, collections and sequences, coroutines, Flow, Java ↔ Kotlin interop and Kotlin Multiplatform.
- Part III — Mastery (4 chapters): compiler errors decoded, a realistic project and study plan, cheat sheets and glossary, and a 20-question self-test.
Made to be skimmed and re-read.
- Dark / light theme — press T or click the toggle; the choice persists.
- Sticky table of contents with scroll-spy and a mobile drawer.
- Syntax highlighting for Java, Kotlin, shell, Gradle and XML, with copy-to-clipboard everywhere.
- Callouts, charts and timelines — tips, warnings and myths kept visually distinct from the text.
- Reading progress bar and expand/collapse all for worked examples.
- Responsive and dependency-free — plain HTML, CSS and JavaScript.
- Format: single-page handbook
- Stack: plain HTML, CSS & JavaScript
- Build step: none for the published page
- Content: 32 chapters · 138 examples · 42 tables
- Extras: 157 callouts · glossary A–Z
- Published: GitHub Pages + custom domain
Who it's for.
Anyone starting out who wants one path instead of two conflicting tutorials — plus people who already write Java and want a structured on-ramp to Kotlin without giving up what Java does well.
Part of a set.
The Rust Handbook follows the same format, so the two read like one series: same structure, same conventions, same self-quiz approach — different language.
Teaching two languages at once is the hard part.
The interesting design problem was deciding what belongs to Java, what belongs to Kotlin and what belongs to both — and being honest about where one is simply better. That constraint is what turned a list of notes into something worth publishing.