142-java-functional-programming

# 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).