Java Application Development

Java
// Spring Boot REST API example
@RestController
@RequestMapping("/api/users")
public class UserController {
    
    @Autowired
    private UserService userService;
    
    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        return userService.findById(id)
            .map(user -> ResponseEntity.ok(user))
            .orElse(ResponseEntity.notFound().build());
    }
    
    @PostMapping
    public ResponseEntity<User> createUser(@Valid @RequestBody User user) {
        User saved = userService.save(user);
        return ResponseEntity.status(HttpStatus.CREATED).body(saved);
    }
}

1. Project Setup

   • Use Maven/Gradle with Spring Boot for new projects
   • Configure application.yml for environment-specific properties
   • Set up logging with SLF4J and Logback

2. Architecture Design

   • Follow layered architecture: Controller → Service → Repository
   • Use dependency injection with @Autowired or constructor injection
   • Implement proper exception handling with @ControllerAdvice

3. Development Process

   • Write unit tests with JUnit 5 and Mockito
   • Use Builder pattern for complex objects
   • Implement validation with Bean Validation annotations

4. Code Quality

   • Follow Java naming conventions
   • Use Optional for null safety
   • Implement proper equals(), hashCode(), and toString()

Progress Checklist:

• Project structure created with proper package organization
• Database entities defined with JPA annotations
• Service layer with business logic implemented
• REST endpoints with proper HTTP status codes
• Unit tests with >80% coverage
• Exception handling configured
• Logging added for debugging and monitoring

Example 1: Service Layer Input: Need user management service Output:

Java
@Service
@Transactional
public class UserService {
    
    private final UserRepository userRepository;
    
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
    
    public Optional<User> findById(Long id) {
        return userRepository.findById(id);
    }
    
    public User save(User user) {
        validateUser(user);
        return userRepository.save(user);
    }
    
    private void validateUser(User user) {
        if (user.getEmail() == null || !user.getEmail().contains("@")) {
            throw new IllegalArgumentException("Valid email required");
        }
    }
}

Example 2: JPA Entity Input: User entity with relationships Output:

Java
@Entity
@Table(name = "users")
public class User {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(nullable = false, unique = true)
    private String email;
    
    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL)
    private List<Order> orders = new ArrayList<>();
    
    // Constructor, getters, setters, equals, hashCode
}

• Use constructor injection over field injection for better testability
• Prefer composition over inheritance for flexibility
• Use enums instead of constants for type safety
• Implement proper logging levels: DEBUG for detailed info, INFO for business events, ERROR for exceptions
• Use try-with-resources for automatic resource management
• Follow SOLID principles especially Single Responsibility and Dependency Inversion
• Use Stream API for functional-style operations on collections
• Implement proper exception hierarchy with custom exceptions extending RuntimeException

• Don't catch and swallow exceptions without proper handling
• Avoid using raw types - always use generics
• Don't use @Autowired on fields in production code - use constructor injection
• Never compare strings with == - use .equals() or Objects.equals()
• Don't ignore compiler warnings - they often indicate real issues
• Avoid creating unnecessary objects in loops - use StringBuilder for string concatenation
• Don't use @Transactional on private methods - it won't work due to proxy limitations
• Never modify collections while iterating - use Iterator.remove() or collect to new collection