142 Java Functional Programming

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.