# 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…

- **Type:** Skill
- **Install:** `agentstack add skill-jabrena-cursor-rules-java-142-java-functional-programming`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [jabrena](https://agentstack.voostack.com/s/jabrena)
- **Installs:** 0
- **Category:** [Cloud & Infrastructure](https://agentstack.voostack.com/c/cloud-infrastructure)
- **Latest version:** 0.1.0
- **License:** Apache-2.0
- **Upstream author:** [jabrena](https://github.com/jabrena)
- **Source:** https://github.com/jabrena/cursor-rules-java/tree/main/skills/142-java-functional-programming
- **Website:** https://jabrena.github.io/cursor-rules-java/

## Install

```sh
agentstack add skill-jabrena-cursor-rules-java-142-java-functional-programming
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# 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

## 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).

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [jabrena](https://github.com/jabrena)
- **Source:** [jabrena/cursor-rules-java](https://github.com/jabrena/cursor-rules-java)
- **License:** Apache-2.0
- **Homepage:** https://jabrena.github.io/cursor-rules-java/

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-jabrena-cursor-rules-java-142-java-functional-programming
- Seller: https://agentstack.voostack.com/s/jabrena
- Browse the marketplace: https://agentstack.voostack.com/browse

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