Java Conventions

Scope

• Language-level rules for Java 25 only — syntax, style, naming, visibility, structure, methods, streams, exceptions, comments.
• Architecture (BCE, layering, packaging) is not in this skill — see bce.
• Build, packaging, framework, and protocol rules are not in this skill — see java-cli-script, java-cli-app, microprofile-server, zb, etc.
• When a composed skill specifies a rule, the composed skill wins; this skill is the fallback baseline.

Java Version & Syntax

• target Java 25; assume all features are GA — never use --enable-preview
• use modern syntax naturally: var, records, sealed types, pattern matching, text blocks, switch expressions
• use var for local variable declarations where the type is obvious from the right-hand side
• use module imports (e.g. import module java.net.http;) over individual type imports
• do not import packages from java.base — it is automatically available
• use switch expressions with arrow syntax (case X -> ...) over old case X: statements with break
• use pattern matching for instanceof — if (o instanceof String s) over cast-and-assign
• use pattern matching in switch for type-based dispatch
• use the diamond operator <> for generic type inference
• never use raw generic types — always parameterize
• prefer void main() / void main(String... args) over public static void main(String[] args); instance main, not static

Java SE APIs

• use Java SE APIs over writing custom code — the standard library has it, usually in java.util, java.nio.file, java.net.http, or java.time
• prefer the most specific Java SE type for the domain — Path over String, URI over String, Duration over long millis, Instant/LocalDate over Date/long

Visibility & Modifiers

• avoid private methods (including private static helpers) — prefer package-private (default) so same-package unit tests can exercise them directly, including edge cases, without round-tripping through the public API
• avoid private fields — prefer package-private so same-package tests can read or seed state without reflection or extra accessors
• reserve private for genuinely sensitive state (credentials, security tokens, invariants that must never be observed externally); the burden of justification is on private, not on package-private
• do not use final on fields — exception: static final for constants like LOGGER
• do not use final on local variables or parameters
• do not use constructor injection — prefer field injection in CDI contexts
• avoid mutable static fields

Interfaces & Classes

• only use interfaces with multiple implementations or for the strategy pattern; never create an interface whose only purpose is to be implemented by one class
• for stateless or procedural logic, prefer interfaces with static methods over classes with private constructors
• in utility interfaces, prefer static over default methods
• avoid anonymous inner classes — extract them into named, testable top-level classes (e.g. a CDI bean produced via @Produces) instead of instantiating an interface inline
• use records by default for value types and data carriers
• use sealed interfaces or sealed classes for closed type hierarchies (pairs well with pattern matching)
• prefer factory methods (static of, from, etc.) in records over passing null to constructors
• prefer composition over inheritance
• create multiple classes only if it decreases complexity and increases readability

Naming

• name classes, modules, and files after their responsibilities, not technical concerns
• avoid meaningless suffixes: *Impl, *Service, *Manager, *Creator
• class names must not end with Control
• reserve protocol- or pattern-specific suffixes for elements that actually fulfill that role: Resource for JAX-RS classes, Factory for actual GoF factories, Builder for classes with method chaining
• avoid the get prefix; use the record-style convention — configuration() not getConfiguration()

Methods & Lambdas

• keep methods short, cohesive, and testable
• create well-named methods for coarse-grained, self-contained logic
• never use multi-statement lambdas — extract them into well-named helper methods
• prefer method references over equivalent lambdas (String::strip over s -> s.strip(), this::isSkillFile over p -> p.endsWith("SKILL.md"))
• extract inline lambda predicates into explaining methods and use method references
• split complex .filter() calls with multiple &&/|| conditions into chained .filter() calls
• extract complex boolean conditions into named predicate methods — write boolean isEligible() instead of inlining age >= 18 && status.equals("active") && !banned
• extract non-trivial calculations into named methods so call sites read as intent, not arithmetic
• do not create empty delegate methods that only forward without added value

Stream & Collections

• prefer java.util.stream.Stream API over for loops
• avoid forEach; prefer terminal operations that return values
• prefer Stream.of over Arrays.stream for known elements
• prefer .toList() over .collect(Collectors.toList())
• prefer List.of / Set.of / Map.of over new ArrayList<>() and array literals for small immutable collections
• avoid creating unnecessary intermediate collections when streaming arrays
• prefer Stream.gather(...) with a Gatherer (Java 24+) over custom Spliterator or stateful forEach for stream transformations that need to keep state across elements or flush a remainder at end-of-stream
• prefer a named intermediate variable over deeply nested method chaining when readability suffers
• return empty collections (List.of(), Set.of(), Map.of()), never null
• do not put null values in collections

Code Style

• KISS and YAGNI — always implement the simplest possible solution that works
• never over-engineer; ask before adding optional features, extension points, or abstractions
• code must be as simple, elegant, and understandable as possible
• always choose the simplest API — prefer higher-level, concise APIs over verbose low-level ones
• prefer multiple simpler lines to one complex line
• use text blocks (""") for all multiline string content (JSON, SQL, HTML, help/usage text, templates) — never +-concatenation or embedded \n escapes
• prefer String.formatted() (instance) over String.format(...) (static) for readability at the call site
• prefer imports over fully qualified class names; remove unused imports
• prefer Files.readString / Files.writeString / Files.lines over BufferedReader/BufferedWriter ceremony
• no blank lines between imports
• use this to reference instance fields when it improves clarity
• prefer enums over plain strings for finite, well-defined values; reuse existing enum constants as values where possible (enum constants do not have to follow naming conventions when reused as values)
• prefer try-with-resources over manual .close() on any AutoCloseable
• extract repeated string literals into named constants — define once, change once
• prefer character literals and named constants over raw numeric literals — write '\n' not 10, define int ESC = '\033' instead of inlining 27
• inline single-use variables — if assigned and used only once on the next line, pass the expression directly
• bind behavior to data with functional fields — store a Runnable, Consumer, or lambda in a record instead of switching on type externally
• separate side effects from conditions — do the work first, then branch on the result; keep the if a pure decision
• use guard clauses (early returns) over deeply nested if/else
• avoid Optional as a parameter type; use Optional as a return type only when absence is a meaningful part of the contract

Exceptions

• prefer unchecked over checked exceptions
• never throw raw java.lang.Exception or RuntimeException directly — throw a specific subclass
• do not re-throw with throw e adding no value
• do not catch and silently ignore exceptions — at minimum, log with context or rethrow wrapped
• use the underscore _ for unused catch parameters (catch (IOException _)) instead of named variables like e or ignored
• create custom exceptions only when they significantly improve robustness or maintainability

Logging

• use java.lang.System.Logger instead of System.out statements
• never use java.util.logging.Logger
• Logger fields must be named LOGGER (uppercase) and marked as static final

HTTP Client

• prefer the synchronous java.net.http.HttpClient APIs
• use asynchronous APIs (HttpClient.sendAsync) only when explicitly requested

Testing

• use AssertJ assertions instead of JUnit assertions
• unit test methods must not start with test or should
• create minimalistic tests first; avoid repetitive or trivial unit tests and keep only essential tests verifying core functionality
• do not write tests for implementations that cannot fail (enums, records, getters/setters)
• generate at most three tests per class under test (applies separately to unit, integration, and system tests)
• the presence of an isEqualTo assertion makes less specific checks (startsWith, isNotNull) obsolete

Comments & JavaDoc

• default to no comments
• only comment when the why is non-obvious: hidden constraint, subtle invariant, workaround for a specific bug, behavior that would surprise a reader
• do not explain what the code does — well-named identifiers do that
• do not write JavaDoc that restates the method signature or rephrases the code
• either describe the why or omit the comment entirely
• when JavaDoc is written, always use Markdown JavaDoc (///, JEP 467); never use HTML-tagged /** */ blocks

Composition with Other Skills

• this skill defines only language-level Java conventions; build, packaging, file layout, frameworks, protocols, and architecture come from the composed skill (e.g. java-cli-script, java-cli-app, microprofile-server, bce, zb)
• when a composed skill adds or refines a rule (e.g. "use IO.println not System.out.println", "no package declaration in single-file scripts", "JAX-RS resources are boundary classes"), apply it on top of these rules; the composed skill always specializes, never contradicts
• if a composed skill is silent on a topic covered here, these rules apply by default