142 Java Functional Programming

Java Functional Programming rules

Identify and apply functional programming principles in Java to improve immutability, expressiveness, and maintainability.

Core areas: Immutable objects and Records (JEP 395), pure functions free of side effects, functional interfaces (Function, Predicate, Consumer, Supplier) and custom @FunctionalInterface types, 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) for concise multi-way conditionals, Stream Gatherers (JEP 461) for custom intermediate operations, effect-boundary separation (side effects at edges, pure core logic), and immutable collections (List.of(), Collectors.toUnmodifiableList()).

Prerequisites: Run ./mvnw compile or mvn compile before applying any changes. If compilation fails, stop immediately — do not proceed until the project compiles successfully. Also verify that the project's maven-compiler-plugin source/target supports the Java features being used.

Multi-step scope: Step 1 validates compilation. Step 2 categorizes functional programming opportunities by impact (CRITICAL, MAINTAINABILITY, PERFORMANCE, EXPRESSIVENESS) and area (immutability violations, side effects, imperative patterns, type safety gaps). Step 3 converts mutable objects to immutable Records or final classes. Step 4 extracts pure functions from methods with side effects. Step 5 replaces imperative loops with declarative Stream API operations. Step 6 adopts Optional for null-safe method returns and functional chaining. Step 7 applies function composition, currying, and higher-order functions. Step 8 introduces sealed types, pattern matching, and switch expressions for type-safe conditional logic. Step 9 hardens collectors (merge functions for toMap, downstream collectors, unmodifiable results). Step 10 runs ./mvnw clean verify to confirm all tests pass after changes.

Before applying changes: Read the reference for detailed good/bad examples, constraints, and safeguards for each functional programming pattern.

Reference

For detailed guidance, examples, and constraints, see references/142-java-functional-programming.md.