Cursor-rules-java 142-java-functional-programming

Use when you need to apply functional programming principles in Java — including writing immutable objects and Records, pure functions, functional interfaces, lambda expressions, Stream API pipelines, Optional for null safety, function composition, higher-order functions, pattern matching for instanceof and switch, sealed classes/interfaces for controlled hierarchies, Stream Gatherers for custom operations, currying/partial application, effect boundary separation, and concurrent-safe functional patterns. Part of the skills-for-java project

install

source · Clone the upstream repo

git clone https://github.com/jabrena/cursor-rules-java

Claude Code · Install into ~/.claude/skills/

T=$(mktemp -d) && git clone --depth=1 https://github.com/jabrena/cursor-rules-java "$T" && mkdir -p ~/.claude/skills && cp -r "$T/skills/142-java-functional-programming" ~/.claude/skills/jabrena-cursor-rules-java-142-java-functional-programming && rm -rf "$T"

manifest: skills/142-java-functional-programming/SKILL.md

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

When to use this skill

Improve the code with Functional Programming
Apply Functional Programming
Refactor the code with Functional Programming

Reference

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