143-java-functional-exception-handling

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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

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Sourcejabrena/cursor-rules-java

Added on2026-03-27

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SKILL.md Content

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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

Improve the code with Functional Exception Handling
Apply Functional Exception Handling
Refactor the code with Functional Exception Handling

Reference

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