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
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- 142-java-functional-programming
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- 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
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- Apache-2.0
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- {"author":"Juan Antonio Breña Moral","version":"0.13.0-SNAPSHOT"}
# 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
- Review Java code for functional programming
- Apply best practices for functional programming in Java code
## Reference
For detailed guidance, examples, and constraints, see [references/142-java-functional-programming.md](references/142-java-functional-programming.md).
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