Java Functional Programming rules

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

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.

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()).

Scope: The reference is organized by examples (with good/bad code patterns) for each core area. Apply recommendations based on applicable examples; validate compilation before changes and run ./mvnw clean verify or mvn clean verify after applying improvements.

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.