JDK 各版本编码实用特性（自用）-阿里西西

// JDK9方式 List<String> list = List.of("Java", "Kotlin", "Scala"); Set<String> set = Set.of("Java", "Kotlin", "Scala"); Map<String, Integer> map = Map.of( "Java", 1995, "Kotlin", 2011, "Scala", 2004 );

// JDK9方式 - 简洁明了 try ( InputStream is = new FileInputStream("source.txt"); OutputStream os = new FileOutputStream("target.txt") ) { is.transferTo(os); // 一行代码搞定 }

// JDK10方式 - 简洁明了 var customersByCity = new HashMap<String, List<Customer>>(); var reader = new BufferedReader(new FileReader("data.txt")); var connection = new URL("https://example.com").openConnection(); // 在for循环中特别有用 for (var entry : customersByCity.entrySet()) { var city = entry.getKey(); var customers = entry.getValue(); // ... } //注： // 1.var只能用于局部变量，不能用于字段、方法参数或返回类型 // 2.声明时必须初始化变量 // 3.不要过度使用，当类型不明显时应该明确声明类型

// 原始集合 List<String> original = new ArrayList<>(List.of("Java", "Kotlin", "Scala")); Set<Integer> originalSet = new HashSet<>(Set.of(1, 2, 3)); Map<String, Integer> originalMap = new HashMap<>(Map.of("one", 1, "two", 2)); // 创建不可修改的副本 List<String> copy = List.copyOf(original); Set<Integer> copiedSet = Set.copyOf(originalSet); Map<String, Integer> copiedMap = Map.copyOf(originalMap); // 修改原集合不影响副本 original.add("Groovy"); System.out.println(original); // [Java, Kotlin, Scala, Groovy] System.out.println(copy); // [Java, Kotlin, Scala] // 尝试修改副本会抛出异常 try { copy.add("Clojure"); // 抛出 UnsupportedOperationException } catch (UnsupportedOperationException e) { System.out.println("Cannot modify immutable copy"); }

// 1. lines() - 按行分割字符串 String multiline = "JavanKotlinnScala"; multiline.lines() .map(String::toUpperCase) .forEach(System.out::println); // 2. strip(), stripLeading(), stripTrailing() - 去除空白字符 String text = " Hello World "; System.out.println(">" + text.strip() + "<"); // >Hello World< System.out.println(">" + text.stripLeading() + "<"); // >Hello World < System.out.println(">" + text.stripTrailing() + "<"); // > Hello World< // strip()与trim()的区别: strip()识别更多的Unicode空白字符 String unicodeWhitespace = "u2005Hellou2005"; System.out.println(">" + unicodeWhitespace.trim() + "<"); // >⠀Hello⠀< System.out.println(">" + unicodeWhitespace.strip() + "<"); // >Hello< // 3. isBlank() - 检查字符串是否为空白 System.out.println(" ".isBlank()); // true System.out.println("".isBlank()); // true System.out.println(" a ".isBlank()); // false // 4. repeat() - 重复字符串 String star = "*"; System.out.println(star.repeat(10)); // ********** System.out.println("=".repeat(20)); // ====================

// 读取文件为String String content = Files.readString(Path.of("config.json")); // 写入String到文件 Files.writeString(Path.of("output.txt"), "Hello Java 11!"); // 使用指定编码 String content = Files.readString(Path.of("data.txt"), StandardCharsets.UTF_8); Files.writeString(Path.of("log.txt"), "Logged at: " + LocalDateTime.now(), StandardCharsets.UTF_8); // 写入字符串集合 List<String> lines = List.of("Line 1", "Line 2", "Line 3"); Files.write(Path.of("lines.txt"), lines);

// 创建简短格式的格式化器 NumberFormat shortFormatter = NumberFormat.getCompactNumberInstance( Locale.US, NumberFormat.Style.SHORT); // 格式化数字 System.out.println(shortFormatter.format(1000)); // 1K System.out.println(shortFormatter.format(1500)); // 2K (四舍五入) System.out.println(shortFormatter.format(1000000)); // 1M System.out.println(shortFormatter.format(1000000000)); // 1B // 长格式 NumberFormat longFormatter = NumberFormat.getCompactNumberInstance( Locale.US, NumberFormat.Style.LONG); System.out.println(longFormatter.format(1000)); // 1 thousand System.out.println(longFormatter.format(1000000)); // 1 million // 其他语言的格式化 NumberFormat germanFormatter = NumberFormat.getCompactNumberInstance( Locale.GERMANY, NumberFormat.Style.SHORT); System.out.println(germanFormatter.format(1000)); // 1.000 NumberFormat chineseFormatter = NumberFormat.getCompactNumberInstance( Locale.CHINA, NumberFormat.Style.SHORT); System.out.println(chineseFormatter.format(1000)); // 1千 System.out.println(chineseFormatter.format(1000000)); // 100万 // 自定义精度 shortFormatter.setMaximumFractionDigits(1); System.out.println(shortFormatter.format(1234)); // 1.2K System.out.println(shortFormatter.format(1500)); // 1.5K

// 传统switch语句 String result; DayOfWeek day = LocalDate.now().getDayOfWeek(); switch (day) { case MONDAY: case TUESDAY: case WEDNESDAY: case THURSDAY: case FRIDAY: result = "Weekday"; break; case SATURDAY: case SUNDAY: result = "Weekend"; break; default: result = "Invalid day"; break; } // 新的switch表达式 String result = switch (day) { case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY -> "Weekday"; case SATURDAY, SUNDAY -> "Weekend"; default -> "Invalid day"; }; // 复杂表达式，带代码块 int numLetters = switch (day) { case MONDAY, FRIDAY, SUNDAY -> { System.out.println("Six letters day"); yield 6; } case TUESDAY -> { System.out.println("Seven letters day"); yield 7; } case THURSDAY, SATURDAY -> { System.out.println("Eight letters day"); yield 8; } case WEDNESDAY -> { System.out.println("Nine letters day"); yield 9; } default -> { throw new IllegalStateException("Invalid day: " + day); } };

// 传统多行字符串 String json = "{n" + " "name": "John Doe",n" + " "age": 30,n" + " "address": {n" + " "street": "123 Main St",n" + " "city": "Anytown"n" + " }n" + "}"; // 使用文本块 String json = """ { "name": "John Doe", "age": 30, "address": { "street": "123 Main St", "city": "Anytown" } } """; // HTML示例 String html = """ <html> <body> <h1>Hello, World!</h1> </body> </html> """; // SQL查询 String query = """ SELECT id, first_name, last_name FROM employees WHERE department_id = ? ORDER BY last_name, first_name """;

// 传统方式 if (obj instanceof String) { String s = (String) obj; if (s.length() > 5) { System.out.println(s.toUpperCase()); } } // 使用模式匹配 if (obj instanceof String s && s.length() > 5) { System.out.println(s.toUpperCase()); } // 在复杂条件中使用 if (obj instanceof String s && s.length() > 10 || obj instanceof List<?> list && list.size() > 5) { // 使用s或list } // 与switch配合使用(JDK17预览特性) Object value = getValue(); switch (value) { case String s when s.length() > 5 -> System.out.println("Long string: " + s); case String s -> System.out.println("Short string: " + s); case List<?> l -> System.out.println("List with " + l.size() + " elements"); default -> System.out.println("Unknown type"); }

// 获取默认的随机数生成器 RandomGenerator random = RandomGenerator.getDefault(); System.out.println(random.nextInt(100)); // 0-99之间的随机数 // 使用特定算法的生成器 RandomGenerator xoroshiro = RandomGenerator.of("Xoroshiro128PlusPlus"); System.out.println(xoroshiro.nextLong()); // 使用L32X64MixRandom - 平衡了速度和质量的算法 RandomGenerator fastRandom = RandomGenerator.of("L32X64MixRandom"); for (int i = 0; i < 5; i++) { System.out.println(fastRandom.nextInt(1000)); } // 创建可复现的随机数序列 (使用相同的种子) RandomGenerator seeded = RandomGenerator.of("Xoshiro256PlusPlus"); ((SplittableRandomGenerator) seeded).setSeed(42); for (int i = 0; i < 5; i++) { System.out.println(seeded.nextInt(100)); } // 生成随机流 DoubleStream randomDoubles = RandomGenerator.getDefault().doubles(1000); randomDoubles.forEach(System.out::println); // 查看所有可用的算法 RandomGenerator.all() .map(provider -> provider.name() + ": " + provider.group()) .sorted() .forEach(System.out::println);

// 创建和启动单个虚拟线程 Thread.startVirtualThread(() -> { System.out.println("Running in virtual thread"); }); // 使用虚拟线程运行多个任务 try (var executor = Executors.newVirtualThreadPerTaskExecutor()) { // 提交1000个任务，每个在独立的虚拟线程中运行 for (int i = 0; i < 1000; i++) { int taskId = i; executor.submit(() -> { System.out.println("Task " + taskId + " running on " + Thread.currentThread()); // 模拟IO操作 try { Thread.sleep(Duration.ofMillis(100)); } catch (InterruptedException e) { Thread.currentThread().interrupt(); } return taskId; }); } } // 自动关闭executor // 以构建器方式创建虚拟线程 ThreadFactory factory = Thread.ofVirtual().name("worker-", 0).factory(); Thread worker = factory.newThread(() -> { // 任务代码 }); worker.start(); // 使用虚拟线程改写传统的阻塞式IO代码 void processFile(Path file) throws IOException { // 这段代码在虚拟线程中运行时不会阻塞平台线程 try (var reader = Files.newBufferedReader(file)) { String line; while ((line = reader.readLine()) != null) { processLine(line); } } }

// 定义一些记录 record Point(int x, int y) {} record Rectangle(Point topLeft, Point bottomRight) {} record Circle(Point center, int radius) {} // 使用传统方式处理记录 Object shape = new Rectangle(new Point(1, 2), new Point(5, 6)); if (shape instanceof Rectangle) { Rectangle r = (Rectangle) shape; Point topLeft = r.topLeft(); Point bottomRight = r.bottomRight(); int width = bottomRight.x() - topLeft.x(); int height = bottomRight.y() - topLeft.y(); System.out.println("Rectangle with width " + width + " and height " + height); } // 使用Record模式解构 if (shape instanceof Rectangle(Point(var x1, var y1), Point(var x2, var y2))) { int width = x2 - x1; int height = y2 - y1; System.out.println("Rectangle with width " + width + " and height " + height); } // 结合switch使用 String getDescription(Object shape) { return switch (shape) { case Rectangle(Point(var x1, var y1), Point(var x2, var y2)) -> "Rectangle from (%d,%d) to (%d,%d)".formatted(x1, y1, x2, y2); case Circle(Point(var x, var y), var r) -> "Circle at (%d,%d) with radius %d".formatted(x, y, r); default -> "Unknown shape"; }; }

// 传统方式 String name = "Alice"; int age = 30; String message = "Hello, " + name + "! Next year, you'll be " + (age + 1) + "."; // 使用字符串模板 String message = STR."Hello, {name}! Next year, you'll be {age + 1}."; // 带表达式的模板 String status = "active"; String message = STR."User status: {status.toUpperCase()} (set {LocalDate.now()})"; // 格式化数字 double value = 1234.56789; String formatted = STR."The value is {value%.2f}"; // "The value is 1234.57" // 多行JSON String json = STR.""" { "name": "{name}", "age": {age}, "isAdult": {age >= 18}, "contacts": [ {generateContactsJson()} ] } """;

// 序列化集合基本用法 SequencedCollection<String> names = new ArrayList<>(List.of("Alice", "Bob", "Charlie")); // 获取第一个和最后一个元素 String first = names.getFirst(); // "Alice" String last = names.getLast(); // "Charlie" // 添加元素到两端 names.addFirst("Zoe"); names.addLast("David"); System.out.println(names); // [Zoe, Alice, Bob, Charlie, David] // 创建反向视图 SequencedCollection<String> reversed = names.reversed(); System.out.println(reversed); // [David, Charlie, Bob, Alice, Zoe] // 序列化Map SequencedMap<String, Integer> scores = new LinkedHashMap<>(); scores.put("Alice", 95); scores.put("Bob", 85); scores.put("Charlie", 90); // 获取第一个和最后一个条目 Map.Entry<String, Integer> firstEntry = scores.firstEntry(); // Alice=95 Map.Entry<String, Integer> lastEntry = scores.lastEntry(); // Charlie=90 // 添加条目到两端 scores.putFirst("Zoe", 100); scores.putLast("David", 80); // 获取键或值的序列化视图 SequencedCollection<String> keys = scores.sequencedKeySet(); SequencedCollection<Integer> values = scores.sequencedValues();