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

1. Compile project before functional refactoring

Run ./mvnw compile or mvn compile and stop immediately if compilation fails.

1. Confirm Java feature compatibility

Verify maven-compiler-plugin source/target supports the functional features planned for adoption.

1. Read functional-programming reference and assess code

Read references/142-java-functional-programming.md and identify opportunities for immutability, pure functions, and functional composition.

1. Apply functional programming improvements

Implement selected functional refactorings while keeping side effects at boundaries.

1. 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).