142-java-functional-programming
This Claude Code skill applies functional programming principles to Java code, covering immutable objects, Records, pure functions, lambda expressions, Stream API pipelines, Optional handling, function composition, pattern matching, sealed classes, and Stream Gatherers. Use it when refactoring code to improve immutability and expressiveness or applying functional paradigms to existing imperative Java implementations, provided the project compiles successfully and the Maven compiler supports the required Java version features.
git clone --depth 1 https://github.com/jabrena/cursor-rules-java /tmp/142-java-functional-programming && cp -r /tmp/142-java-functional-programming/skills/142-java-functional-programming ~/.claude/skills/142-java-functional-programmingSKILL.md
# Java Functional Programming rules Identify and apply functional programming principles in Java to improve immutability, expressiveness, and maintainability. **What is covered in this Skill?** - Immutable objects and Records (JEP 395) - Pure functions free of side effects - Functional interfaces: `Function`, `Predicate`, `Consumer`, `Supplier`, custom `@FunctionalInterface` - Lambda expressions and method references - Stream API: filter/map/reduce pipelines, parallel streams, `toUnmodifiable*` collectors - `Optional` idiomatic usage: `map`/`flatMap`/`filter`/`orElse*` over `isPresent()`+`get()` - Function composition: `andThen`/`compose` - Higher-order functions: memoization, currying, partial application - Pattern Matching for `instanceof` and `switch` (Java 21) - Sealed classes and interfaces (Java 17) for exhaustive domain hierarchies - Switch Expressions (Java 14), Stream Gatherers (JEP 461) - Effect-boundary separation: side effects at edges, pure core logic - Immutable collections: `List.of()`, `Collectors.toUnmodifiableList()` **Scope:** The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples. ## Constraints Before applying any functional programming changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until the project compiles successfully. Verify that maven-compiler-plugin source/target supports the Java features being used. - **MANDATORY**: Run `./mvnw compile` or `mvn compile` before applying any changes - **SAFETY**: If compilation fails, stop immediately — do not proceed until the project compiles successfully - **VERIFY**: Verify maven-compiler-plugin source/target supports the Java features being used - **VERIFY**: Run `./mvnw clean verify` or `mvn clean verify` after applying improvements - **BEFORE APPLYING**: Read the reference for detailed good/bad examples, constraints, and safeguards for each functional programming pattern - **EDGE CASE**: If request scope is ambiguous, stop and ask a clarifying question before applying changes - **EDGE CASE**: If required inputs, files, or tooling are missing, report what is missing and ask whether to proceed with setup guidance ## When to use this skill - Improve the code with Functional Programming - Apply Functional Programming - Refactor the code with Functional Programming ## Workflow 1. **Compile project before functional refactoring** Run `./mvnw compile` or `mvn compile` and stop immediately if compilation fails. 2. **Confirm Java feature compatibility** Verify maven-compiler-plugin source/target supports the functional features planned for adoption. 3. **Read functional-programming reference and assess code** Read `references/142-java-functional-programming.md` and identify opportunities for immutability, pure functions, and functional composition. 4. **Apply functional programming improvements** Implement selected functional refactorings while keeping side effects at boundaries. 5. **Verify with full build** Run `./mvnw clean verify` or `mvn clean verify` after applying improvements. ## Reference For detailed guidance, examples, and constraints, see [references/142-java-functional-programming.md](references/142-java-functional-programming.md).
Use when you need to generate a checklist document with Java system prompts, following the embedded template exactly and producing INVENTORY-SKILLS-JAVA.md in the project root. This should trigger for requests such as Create Java system prompts checklist; Generate INVENTORY-SKILLS-JAVA.md; Use @001-skills-inventory. Part of cursor-rules-java project
Use when you need to generate a checklist document with embedded agents inventory, following the embedded template exactly and producing INVENTORY-AGENTS-JAVA.md in the project root. This should trigger for requests such as Create embedded agents inventory checklist; Generate INVENTORY-AGENTS-JAVA.md; Use @002-agents-inventory. Part of cursor-rules-java project
Use when you need to install the embedded robot agents into either .cursor/agents or .claude/agents, selecting the destination interactively and copying the embedded agent definitions from project assets. This should trigger for requests such as Install embedded agents; Bootstrap .cursor/agents; Bootstrap .claude/agents; Copy robot agents. Part of cursor-rules-java project
Guides the creation of agile epics with comprehensive definition including business value, success criteria, and breakdown into user stories. Use when the user wants to create an agile epic, define large bodies of work, break down features into user stories, or document strategic initiatives. This should trigger for requests such as Create an agile epic; Write an epic; I need to create an epic; Define an epic; Epic definition. Part of cursor-rules-java project
Guides the creation of detailed agile feature documentation from an existing epic. Use when the user wants to split an epic into feature files, derive features with scope and acceptance criteria, or plan feature documentation for stakeholders or engineering. This should trigger for requests such as Create features from an epic; Split epic into features; Feature files from epic; Derive features from epic. Part of cursor-rules-java project
Guides the creation of agile user stories and Gherkin feature files. Use when the user wants to create a user story, write acceptance criteria, define Gherkin scenarios, or author BDD feature files. This should trigger for requests such as Create a user story; Write a user story; I need to write a user story. Part of cursor-rules-java project
Use when you need to generate Architecture Decision Records (ADRs) for a Java project through an interactive, conversational process that systematically gathers context, stakeholders, options, and outcomes to produce well-structured ADR documents. This should trigger for requests such as Generate ADR; Create Architecture Decision Record; Document architecture decision; Architecture Decision Record for Java. Part of cursor-rules-java project
Facilitates conversational discovery to create Architectural Decision Records (ADRs) for functional requirements covering CLI, REST/HTTP APIs, or both. Use when the user wants to document command-line or HTTP service architecture, capture functional requirements, create ADRs for CLI or API projects, or design interfaces with documented decisions. This should trigger for requests such as Create ADR for functional requirements; Document functional requirements; Capture functional requirements; Generate functional requirements in an ADR. Part of cursor-rules-java project