Java Coding Standards

# Java Coding Standards

Standards for readable, maintainable Java (17+) code in Spring Boot and Quarkus services.

## When to Use

- Writing or reviewing Java code in Spring Boot or Quarkus projects
- Enforcing naming, immutability, or exception handling conventions
- Working with records, sealed classes, or pattern matching (Java 17+)
- Reviewing use of Optional, streams, or generics
- Structuring packages and project layout
- **[QUARKUS]**: Working with CDI scopes, Panache entities, or reactive pipelines

## How It Works

### Framework Detection

Before applying standards, determine the framework from the build file:

- Build file contains `quarkus` → apply **[QUARKUS]** conventions
- Build file contains `spring-boot` → apply **[SPRING]** conventions
- Neither detected → apply shared conventions only

## Core Principles

- Prefer clarity over cleverness
- Immutable by default; minimize shared mutable state
- Fail fast with meaningful exceptions
- Consistent naming and package structure
- **[QUARKUS]**: Favor build-time over runtime processing; avoid runtime reflection where possible

## Examples

The sections below show concrete Spring Boot, Quarkus, and shared Java examples
for naming, immutability, dependency injection, reactive code, exceptions,
project layout, logging, configuration, and tests.

## Naming

```java
// PASS: Classes/Records: PascalCase
public class MarketService {}
public record Money(BigDecimal amount, Currency currency) {}

// PASS: Methods/fields: camelCase
private final MarketRepository marketRepository;
public Market findBySlug(String slug) {}

// PASS: Constants: UPPER_SNAKE_CASE
private static final int MAX_PAGE_SIZE = 100;

// PASS: [QUARKUS] JAX-RS resources named as *Resource, not *Controller
public class MarketResource {}

// PASS: [SPRING] REST controllers named as *Controller
public class MarketController {}
```

## Immutability

```java
// PASS: Favor records and final fields
public record MarketDto(Long id, String name, MarketStatus status) {}

public class Market {
  private final Long id;
  private final String name;
  // getters only, no setters
}

// PASS: [QUARKUS] Panache active-record entities use public fields (Quarkus convention)
@Entity
public class Market extends PanacheEntity {
  public String name;
  public MarketStatus status;
  // Panache generates accessors at build time; public fields are idiomatic here
}

// PASS: [QUARKUS] Panache MongoDB entities
@MongoEntity(collection = "markets")
public class Market extends PanacheMongoEntity {
  public String name;
  public MarketStatus status;
}
```

## Optional Usage

```java
// PASS: Return Optional from find* methods
// [SPRING]
Optional<Market> market = marketRepository.findBySlug(slug);

// [QUARKUS] Panache
Optional<Market> market = Market.find("slug", slug).firstResultOptional();

// PASS: Map/flatMap instead of get()
return market
    .map(MarketResponse::from)
    .orElseThrow(() -> new EntityNotFoundException("Market not found"));
```

## Streams Best Practices

```java
// PASS: Use streams for transformations, keep pipelines short
List<String> names = markets.stream()
    .map(Market::name)
    .filter(Objects::nonNull)
    .toList();

// FAIL: Avoid complex nested streams; prefer loops for clarity
```

## Dependency Injection

```java
// PASS: [SPRING] Constructor injection (preferred over @Autowired on fields)
@Service
public class MarketService {
  private final MarketRepository marketRepository;

  public MarketService(MarketRepository marketRepository) {
    this.marketRepository = marketRepository;
  }
}

// PASS: [QUARKUS] Constructor injection
@ApplicationScoped
public class MarketService {
  private final MarketRepository marke