Java OOP Quick Reference Cheat Sheet

// Class with encapsulation
public class User {
    private final String name;
    public User(String name) { this.name = name; }
    public String getName() { return name; }
}

// Interface
public interface Repository<T> {
    Optional<T> findById(Long id);
    List<T> findAll();
}

// Record (Java 16+)
public record Point(int x, int y) {}

// Optional chaining
String name = user.map(User::getName)
    .filter(n -> !n.isBlank())
    .orElse("Anonymous");

// Builder pattern
User user = User.builder()
    .name("Alice")
    .email("alice@example.com")
    .role(Role.ADMIN)
    .build();

// Try-with-resources
try (var reader = new BufferedReader(new FileReader(path))) {
    reader.lines().forEach(System.out::println);
}

// Custom exception hierarchy
public class AppException extends RuntimeException {
    private final ErrorCode code;
    public AppException(ErrorCode code, String msg) {
        super(msg);
        this.code = code;
    }
}

// Checked vs unchecked
try {
    riskyOperation();
} catch (IOException e) {
    throw new AppException(ErrorCode.IO, e.getMessage());
} finally {
    cleanup();
}

// Generics with bounds
public <T extends Comparable<T>> T max(T a, T b) {
    return a.compareTo(b) >= 0 ? a : b;
}

// Sealed classes (Java 17+)
public sealed interface Shape
    permits Circle, Rectangle {
    double area();
}

public record Circle(double radius) implements Shape {
    public double area() { return Math.PI * radius * radius; }
}

public record Rectangle(double w, double h) implements Shape {
    public double area() { return w * h; }
}

@Test
void shouldFindUser() {
    var user = service.findById(1L);
    assertThat(user).isPresent();
    assertThat(user.get().getName()).isEqualTo("Alice");
}

@ParameterizedTest
@ValueSource(strings = {"", " ", "  "})
void shouldRejectBlankNames(String name) {
    assertThrows(ValidationException.class,
        () -> new User(name));
}

@Test
void shouldHandleConcurrency() throws Exception {
    var latch = new CountDownLatch(10);
    // ...
}