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
- Refactor Java code to use streams or Optionals safely
- Improve immutability and pure functions in Java
Workflow
- Compile project before functional refactoring
Run ./mvnw compile or mvn compile and stop immediately if compilation fails.
- Confirm Java feature compatibility
Verify maven-compiler-plugin source/target supports the functional features planned for adoption.
- Read functional-programming reference and assess code
Read references/142-java-functional-programming.md and identify opportunities for immutability, pure functions, and functional composition.
- Apply functional programming improvements
Implement selected functional refactorings while keeping side effects at boundaries.
- 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.