| 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. Part of the skills-for-java project |
| license | Apache-2.0 |
| metadata | {"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.
