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
- Refactor Java code to use streams or Optionals safely
- Improve immutability and pure functions in Java

## 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).