《Java 编程入门》附录 B 标准库地图

List<Book> books = new ArrayList<>();
books.add(new Book("978-7-111-1", "Java 入门"));

Map<String, Book> byIsbn = books.stream()
    .collect(Collectors.toMap(Book::isbn, b -> b));

Optional<Book> found = Optional.ofNullable(byIsbn.get("978-7-111-1"));

Function<String, Integer> length = String::length;
Predicate<Book> available = b -> b.status() == LoanStatus.AVAILABLE;
Supplier<List<Book>> empty = ArrayList::new;

Map<String, List<Book>> byAuthor = books.stream()
    .collect(Collectors.groupingBy(Book::author));

String titles = books.stream()
    .map(Book::title)
    .collect(Collectors.joining("，", "《", "》"));

try (BufferedReader reader = new BufferedReader(
        new InputStreamReader(new FileInputStream("books.csv"), StandardCharsets.UTF_8))) {
    String line;
    while ((line = reader.readLine()) != null) {
        // ...
    }
}

Path dir = Path.of("data");
Files.createDirectories(dir);

Path file = dir.resolve("books.csv");
Files.writeString(file, "isbn,title\n", StandardCharsets.UTF_8);

try (Stream<Path> paths = Files.walk(dir)) {
    paths.filter(Files::isRegularFile).forEach(System.out::println);
}

LocalDate today = LocalDate.now();
LocalDate due = today.plusDays(14);
boolean overdue = LocalDate.now().isAfter(due);

DateTimeFormatter fmt = DateTimeFormatter.ISO_LOCAL_DATE;
String text = due.format(fmt);

BigDecimal price = new BigDecimal("39.90");
BigDecimal total = price.multiply(new BigDecimal("3"));
BigDecimal rounded = total.setScale(2, RoundingMode.HALF_UP);

HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://example.com/api/books"))
    .header("Accept", "application/json")
    .GET()
    .build();

HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(response.statusCode());
System.out.println(response.body());

try (ExecutorService pool = Executors.newFixedThreadPool(4)) {
    Future<Integer> future = pool.submit(() -> 1 + 1);
    System.out.println(future.get());
}

Pattern isbn = Pattern.compile("^97[89]-\\d-\\d{3}-\\d{5}-\\d$");
boolean valid = isbn.matcher("978-7-111-12345-6").matches();

Class<?> type = Book.class;
for (Method method : type.getDeclaredMethods()) {
    System.out.println(method.getName());
}

MemoryMXBean memory = ManagementFactory.getMemoryMXBean();
System.out.println(memory.getHeapMemoryUsage());

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Audited {
    String value() default "";
}