Java Skill

Overview

Modern Java is expressive and powerful. This skill guides writing clean, maintainable Java that leverages modern language features and frameworks properly.

Core principle: Favor immutability and composition. Modern Java (17+) provides the tools—records, sealed classes, pattern matching—use them.

The Java Development Process

Phase 1: Choose Modern Patterns

Before writing implementation:

1. Use Records for Data

   • Immutable data classes
   • Built-in equals, hashCode, toString
   • No boilerplate

2. Handle Absence with Optional

   • Return type only, not parameters
   • Chain operations, don't check .isPresent()

3. Design for Immutability

   • Final fields by default
   • Defensive copies for collections
   • Builder pattern for complex objects

Phase 2: Implement Cleanly

Write modern Java:

1. Use Records for DTOs

   // ✅ Modern: Record
   public record User(String name, String email, int age) {}
   
   // ❌ Legacy: Mutable POJO
   public class User {
       private String name;
       private String email;
       // getters, setters, equals, hashCode, toString...
   }
   

2. Chain Optional Operations

   // ✅ Functional style
   String email = findUser(id)
       .map(User::email)
       .orElse("unknown@example.com");
   
   // ❌ Imperative style
   Optional<User> user = findUser(id);
   String email;
   if (user.isPresent()) {
       email = user.get().email();
   } else {
       email = "unknown@example.com";
   }
   

3. Use Pattern Matching

   // ✅ Switch expressions
   String result = switch (status) {
       case ACTIVE -> "Active";
       case PENDING -> "Pending";
       case INACTIVE -> "Inactive";
   };
   
   // ✅ Pattern matching for instanceof
   if (shape instanceof Circle c) {
       return Math.PI * c.radius() * c.radius();
   }
   

Phase 3: Review for Quality

Before approving:

1. Check Immutability

   • Records used for data?
   • Collections defensively copied?
   • No unnecessary mutability?

2. Check Null Safety

   • Optional used appropriately?
   • No null returns from methods?
   • Null checks at boundaries only?

3. Check Spring Patterns

   • Constructor injection used?
   • Transactions at service layer?
   • No business logic in controllers?

Red Flags - STOP and Fix

Optional Red Flags

// Optional.get() without check
String name = optional.get();  // NoSuchElementException!

// Optional as parameter
void process(Optional<String> name) {  // Just use @Nullable

// Optional of nullable
Optional.of(nullableValue);  // NullPointerException!

// isPresent() + get() pattern
if (opt.isPresent()) {
    return opt.get();  // Use orElse, map, etc.
}

Dependency Injection Red Flags

// Field injection
@Autowired
private UserRepository userRepository;  // Hard to test!

// Circular dependencies
@Service class A { @Autowired B b; }
@Service class B { @Autowired A a; }  // Design problem

// @Autowired on setter
@Autowired
public void setRepository(Repo repo) {}  // Use constructor

Design Red Flags

// Mutable state in services
@Service
public class UserService {
    private List<User> cache = new ArrayList<>();  // Shared mutable state!
}

// Null return values
public User findById(Long id) {
    return repo.findById(id).orElse(null);  // Return Optional instead
}

// Checked exceptions for flow control
try {
    return findUser(id);
} catch (UserNotFoundException e) {
    return createUser(id);  // Use Optional
}

Common Rationalizations - Don't Accept These

Java Quality Checklist

Before approving Java code:

• Modern features: Records, pattern matching, switch expressions
• Immutability: Final fields, defensive copies
• Optional correct: No .get(), no parameters, chained operations
• Constructor injection: No @Autowired on fields
• No nulls: Optional returns, validated inputs
• Tests present: Unit and integration tests
• Linting clean: SpotBugs, Checkstyle pass

Quick Patterns

Service Layer

@Service
@RequiredArgsConstructor  // Lombok generates constructor
public class UserService {

    private final UserRepository userRepository;
    private final EventPublisher eventPublisher;

    @Transactional(readOnly = true)
    public Optional<User> findById(Long id) {
        return userRepository.findById(id);
    }

    @Transactional
    public User create(CreateUserRequest request) {
        User user = User.builder()
            .name(request.name())
            .email(request.email())
            .build();

        User saved = userRepository.save(user);
        eventPublisher.publish(new UserCreatedEvent(saved));
        return saved;
    }
}

Controller Layer

@RestController
@RequestMapping("/api/users")
@RequiredArgsConstructor
public class UserController {

    private final UserService userService;

    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        return userService.findById(id)
            .map(ResponseEntity::ok)
            .orElse(ResponseEntity.notFound().build());
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public User createUser(@Valid @RequestBody CreateUserRequest request) {
        return userService.create(request);
    }
}

Exception Handling

@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(ResourceNotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    public ErrorResponse handleNotFound(ResourceNotFoundException ex) {
        return new ErrorResponse(ex.getMessage());
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public ErrorResponse handleValidation(MethodArgumentNotValidException ex) {
        List<String> errors = ex.getBindingResult()
            .getFieldErrors()
            .stream()
            .map(e -> e.getField() + ": " + e.getDefaultMessage())
            .toList();
        return new ErrorResponse("Validation failed", errors);
    }
}

Quick Commands

# Build
mvn clean install
./gradlew build

# Test
mvn test
./gradlew test

# Static analysis
mvn spotbugs:check
mvn checkstyle:check

# Coverage report
mvn jacoco:report

# Run application
mvn spring-boot:run
./gradlew bootRun

References

Detailed patterns and examples in references/:

• modern-java.md - Java 17+ features
• spring-patterns.md - Spring Boot best practices
• testing-patterns.md - JUnit 5 and Mockito patterns