Java Practice

Optional language. Java is not one of the languages this curriculum targets, so these exercises have no reference implementations yet and are excluded from ol verify build and from CI. They are kept because the exercises are worth doing, and every ol command works on them by name.

Toolchain: mise, with a .mise.toml pin. On a chezmoi-managed machine it comes from the install_alt_langs flag, which is off by default.

macOS ships /usr/bin/java as a stub that exists and only prints “Unable to locate a Java Runtime”, so check with javac -version, not command -v javac.

Master JVM internals, generics, concurrency, and modern Java features.

Language Essentials

Concept	Details
Memory Model	JVM GC (G1/ZGC), heap/stack, escape analysis, value types (Valhalla preview)
Type System	Strong static, generics (type erasure), sealed classes, records
Error Handling	Checked + unchecked exceptions, Optional, try-with-resources
Concurrency	Virtual threads (21+), CompletableFuture, java.util.concurrent, structured concurrency
Build	gradle or maven, jshell for REPL
Standard	Java 21+ LTS

Key Idioms

• Records for immutable data transfer objects
• Sealed interfaces + pattern matching (switch expressions)
• Virtual threads over thread pools for I/O-bound work
• Stream API for functional collection processing
• Try-with-resources for AutoCloseable cleanup
• Optional instead of null returns
• var for local variable type inference

Reference Documentation

• Java Language Specification — authoritative
• Java SE API Docs — standard library
• Dev.java — official tutorials
• Effective Java (Bloch, 3rd ed.) — essential patterns
• Java Concurrency in Practice (Goetz) — concurrency bible

Practising these

practice/bin/ol list  build java          # which of these have references yet
practice/bin/ol start build java 01       # clone it stubbed into .scratchpad/
practice/bin/ol check build java 01       # run it; exit code is the verdict
practice/bin/ol diff  build java 01       # your attempt against the reference

Exercises are ordered by concept, not by difficulty, so pick by the stars rather than by the number. An exercise listed below with no reference yet is a row in this table and nothing more; see the practice path for how the harness works.

Exercises

#	Exercise	Concepts	Difficulty
01	ArrayList<T> from scratch	Generics, type erasure, Iterable	⭐⭐
03	Virtual thread HTTP server	Virtual threads, ServerSocket, structured concurrency	⭐⭐⭐
04	Dependency injection (mini-Spring)	Reflection, annotations, Proxy	⭐⭐⭐⭐
05	Stream collector library	Collector interface, Spliterator, parallel streams	⭐⭐⭐
06	Event sourcing framework	Records, sealed interfaces, pattern matching	⭐⭐⭐⭐
07	Garbage collector simulator	Memory model, reference types, mark-sweep	⭐⭐⭐⭐
08	Bytecode interpreter (subset)	ClassFile format, operand stack, JVM internals	⭐⭐⭐⭐⭐
09	Reactive streams publisher	Flow API, backpressure, SubmissionPublisher	⭐⭐⭐⭐
10	Annotation processor	javax.annotation.processing, code generation	⭐⭐⭐⭐

Exercise 02 (concurrent hash map) is consolidated into the course modules ds.02 and ds.05 (Systems Data Structures); the row had no reference.