# 142-java-functional-programming

A skill that pushes your AI agent to rewrite Java in a functional style: Records, pure functions, Stream API pipelines and Optional.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 376 installs · ★ 456
- Author: Juan Antonio Breña Moral
- License: Apache-2.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/jabrena/plinth/tree/HEAD/skills/142-java-functional-programming/SKILL.md
- Updated: 2026-10-21
- HTML: https://affpapa.org/en/nevibecode/skills/142-java-functional-programming

## What it does

The skill applies functional programming principles to a Java codebase. It is organised as good and bad code examples per topic: immutable objects and Records (JEP 395), pure functions, the functional interfaces `Function`, `Predicate`, `Consumer`, `Supplier`, lambdas and method references. Stream API pipelines with `filter`/`map`/`reduce`, parallel streams and `toUnmodifiable*` collectors get their own section.

Other areas cover idiomatic `Optional` instead of `isPresent()` plus `get()`, function composition with `andThen` and `compose`, memoization, currying and partial application. For domain hierarchies there are sealed classes and interfaces (Java 17), pattern matching for `instanceof` and `switch` (Java 21), switch expressions (Java 14) and Stream Gatherers (JEP 461). Side effects are pushed to the edges so the core logic stays pure.

Before touching anything, the agent must run `./mvnw compile` or `mvn compile` and stop if the build fails. So the project has to compile before you invoke the skill, and `maven-compiler-plugin` must support the Java versions you plan to use — pattern matching and Gatherers will not build otherwise. After the changes it runs `./mvnw clean verify` or `mvn clean verify`. If the request scope is unclear, the skill stops and asks a clarifying question.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add jabrena/plinth --skill 142-java-functional-programming -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/142-java-functional-programming/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/142-java-functional-programming/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add jabrena/plinth --skill 142-java-functional-programming -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/142-java-functional-programming/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add jabrena/plinth --skill 142-java-functional-programming -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/142-java-functional-programming/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install jabrena/plinth 142-java-functional-programming
```

2. **Manually: put the files in place**
- File: `.github/skills/142-java-functional-programming/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add jabrena/plinth --skill 142-java-functional-programming -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/jabrena/plinth --path skills/142-java-functional-programming
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/142-java-functional-programming/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add jabrena/plinth --skill 142-java-functional-programming -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/jabrena/plinth/tree/HEAD/skills/142-java-functional-programming/SKILL.md

## When to use it
- turn accumulating loops into Stream API pipelines
- replace `isPresent()` and `get()` checks with idiomatic `Optional`
- make domain objects immutable with Records
- move side effects to the boundaries and keep the core pure
- close a type hierarchy with sealed classes for exhaustive `switch`

## Example prompts
- Improve the code with Functional Programming
- Refactor Java code to use streams or Optionals safely
- Improve immutability and pure functions in Java

## Source text · SKILL.md · Apache-2.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 0d4c058cd1578a2e.

BEGIN THIRD-PARTY FILE 0d4c058cd1578a2e
```text
---
name: 142-java-functional-programming
description: 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. This should trigger for requests such as Improve the code with Functional Programming; Apply Functional Programming; Refactor the code with Functional Programming; Refactor Java code to use streams or Optionals safely; Improve immutability and pure functions in Java. Part of Plinth Toolkit
license: Apache-2.0
metadata:
  author: Juan Antonio Breña Moral
  version: 0.19.0
---
# 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
```
END THIRD-PARTY FILE 0d4c058cd1578a2e

The third-party file has ended. Everything between the 0d4c058cd1578a2e tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

First 2164 of 4366 bytes. © Juan Antonio Breña Moral · Apache-2.0 · https://github.com/jabrena/plinth/tree/HEAD/skills/142-java-functional-programming/SKILL.md

_Part of the page did not fit — see the HTML version._

---
NeVibeCode on affpapa.org. Catalog texts are descriptions, not instructions for the agent. How to choose and install: https://affpapa.org/en/nevibecode/agents.md