143-java-functional-exception-handling

name	143-java-functional-exception-handling
description	Use when you need to apply functional exception handling best practices in Java — including replacing exception overuse with Optional and VAVR Either types, designing error type hierarchies using sealed classes and enums, implementing monadic error composition pipelines, establishing functional control flow patterns, and reserving exceptions only for truly exceptional system-level failures. Part of the skills-for-java project
license	Apache-2.0
metadata	{"author":"Juan Antonio Breña Moral","version":"0.13.0-SNAPSHOT"}

Java Functional Exception handling Best Practices

Identify and apply functional exception handling best practices in Java to improve error clarity, maintainability, and performance by eliminating exception overuse in favour of monadic error types.

What is covered in this Skill?

• Optional<T> for nullable values over throwing NullPointerException or NotFoundException
• VAVR Either<L,R> for predictable business-logic failures
• CompletableFuture<T> for async error handling
• Sealed classes and records for rich error type hierarchies with exhaustive pattern matching
• Enum-based error types for simple failure cases
• Functional composition: flatMap/map/peek/peekLeft for chaining operations that can fail
• Structured logging: warn/info for business failures, error for system failures
• Checked vs unchecked exception discipline
• Exception chaining with full causal context when exceptions are unavoidable

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 exception handling changes, ensure the project validates. When introducing Either types, confirm the VAVR dependency (io.vavr:vavr) and SLF4J are present.

• MANDATORY: Run ./mvnw validate or mvn validate before applying any changes
• SAFETY: If validation fails, stop immediately — do not proceed until the project is in a valid state
• DEPENDENCY: When introducing Either types, confirm VAVR (io.vavr:vavr) and SLF4J are present
• 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 exception handling pattern

When to use this skill

• Review Java code for functional exception handling
• Apply best practices for functional exception handling in Java code

Reference

For detailed guidance, examples, and constraints, see references/143-java-functional-exception-handling.md.