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**...