java11新特性簡單介紹

本地變量類型推斷

public class Client {
  public static void main(String[] args) {
    var name = "lisi";
    System.out.println(name);

    List<String> names = Arrays.asList("a", "b", "c");
    for (var s : names) {
      System.out.println(s);
    }
    for (var i = 0; i < names.size(); i++) {
      System.out.println(names.get(i));
    }
  }
}

編譯器會根據右邊的表達式自動推斷左邊的類型，

var name = "lisi";
String name = "lisi";

這兩個聲明是一樣的。

字符串增強

之前版本的字符串是使用字符數組存儲的，新版本換成了字節數組存儲，增加了一些新方法

public class Client {
  public static void main(String[] args) {
// 判斷是否為空白
    System.out.println("  ".isBlank());
// 去除左邊和右邊的空白
    System.out.println(" lisi ".strip().length()); 
// 去除左邊的空白
    System.out.println(" lisi ".stripLeading().length());
// 去除右邊的空白
    System.out.println(" lisi ".stripTrailing().length());
// 重復字符串
    System.out.println("lisi".repeat(3).length());
// 行數統計
    System.out.println("lisi\nlisi\n".lines().count());
  }
}

集合增強

List，Set，Map都添加了of和copyOf方法來創建不可變的集合。

public class Client {
  public static void main(String[] args) {
    List<String> list = List.of("AAA", "BBB");
    List<String> copyList = List.copyOf(list);
    System.out.println(list == copyList); // true
    List<String> a = Arrays.asList("a");
    System.out.println(List.copyOf(a) == a); // false
    Set<String> set = Set.of("AAA", "BBB");
    Set<String> copySet = Set.copyOf(set);
    System.out.println(copySet == set);
    Map<String, Object> map = Map.of("name", "lisi", "age", 23);
    System.out.println(Map.copyOf(map) == map);
    System.out.println(Arrays.toString(list.toArray(String[]::new)));
  }
}

copyOf的實現原理

// make a copy, short-circuiting based on implementation class
    @SuppressWarnings("unchecked")
    static <E> List<E> listCopy(Collection<? extends E> coll) {
        if (coll instanceof AbstractImmutableList && coll.getClass() != SubList.class) {
            return (List<E>)coll;
        } else {
            return (List<E>)List.of(coll.toArray());
        }
    }

如果已經是不可變集合，直接返回自身，否則返回一個新的不可變集合。
Collection 新加了一個默認方法toArray(IntFunction)

// JDK11之前
    System.out.println(Arrays.toString(list.toArray(new String[0])));
// JDK11
    System.out.println(Arrays.toString(list.toArray(String[]::new)));

Stream 增強

public class Client {
  public static void main(String[] args) {
// null 就是空stream
    System.out.println(Stream.ofNullable(null).count());
    List<String> list = List.of("a", "b", "c");
// 截止到第一個不滿足條件的元素
    list.stream().takeWhile(x -> x.equals("a")).forEach(System.out::println);
    System.out.println("==========");
// 從第一個不滿足條件的元素開始
    list.stream().dropWhile(x -> x.equals("a")).forEach(System.out::println);
    System.out.println("==========");
// 迭代 直到不滿足條件
    Stream.iterate(3, x -> x < 7, x -> x + 1).forEach(System.out::println);
  }
}

Optional 增強

public class Client {
  public static void main(String[] args) {
    Optional.of("abc").or(() -> Optional.of("hello")).stream().forEach(System.out::println);
  }
}

增加了or(Supplier<? extends Optional<? extends T>>)和stream()方法

InputStream 增強

public class Client {
  public static void main(String[] args) {
    try {
      try (InputStream is = Client.class.getClassLoader().getResourceAsStream("html.txt");
           ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
        is.transferTo(baos);
        System.out.println(new String(baos.toByteArray()));
      }
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

增加了transferTo(OutputStream) 方法，將數據輸出到OutputStream。

Http Client API

public class Client {
  public static void main(String[] args) throws IOException, InterruptedException {
// 創建http客戶端
    HttpClient client = HttpClient.newHttpClient();
// 創建GET請求
    HttpRequest request = HttpRequest.newBuilder()
        .GET()
        .uri(URI.create("https://www.baidu.com"))
        .build();
// 同步發送請求
    System.out.println(client.send(request, HttpResponse.BodyHandlers.ofString()).body());
    System.out.println("============");
// 異步發送請求
    client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
        .thenApply(HttpResponse::body)
        .thenAccept(System.out::println)
        .join();

// 創建POST請求
    HttpRequest postRequest = HttpRequest.newBuilder()
        .uri(URI.create("https://postman-echo.com/post"))
        .header("Content-Type", "application/json")
        .POST(HttpRequest.BodyPublishers.ofString(JSON.toJSONString(Map.of("method", "POST"))))
        .build();
    System.out.println(client.send(postRequest, HttpResponse.BodyHandlers.ofString()).body());

  }
}

java 命令編譯運行源碼

java Client.java

一個命令編譯加運行