Java全版本特性与用法示例(含总览 + 分版本要点)

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello");
    }
}

// JDBC 1.0 起：用反射加载驱动是常见写法
// Class.forName("org.example.Driver");
// java.sql.Connection c = java.sql.DriverManager.getConnection("jdbc:...");

class Outer {
    class Inner { void f() { System.out.println(Outer.this); } }
}

import java.util.*;

List list = new ArrayList();
list.add("a");
for (Iterator it = list.iterator(); it.hasNext(); ) {
    String s = (String) it.next();
}
Map map = new HashMap();
map.put("k", "v");

import javax.swing.JFrame;
import javax.swing.JLabel;
// JFrame f = new JFrame(); f.getContentPane().add(new JLabel("ok")); f.pack(); f.setVisible(true);

import java.lang.reflect.*;

interface I { int run(); }
class H implements InvocationHandler {
    public Object invoke(Object p, Method m, Object[] a) { return 1; }
}
// I r = (I) Proxy.newProxyInstance(I.class.getClassLoader(), new Class[]{I}, new H);

class A4 {
    void m(int x) {
        assert x >= 0 : "x must be non-negative";
    }
}

import java.util.*;

enum Day { MON, TUE }
List<String> list = new ArrayList<String>();
for (String s : list) { }
void foo(int... xs) { }

import javax.script.*;
ScriptEngine eng = new ScriptEngineManager().getEngineByName("js");
if (eng != null) {
    eng.eval("1 + 2");
}

import java.io.InputStream;
import java.nio.file.*;
import java.util.*;
import java.util.concurrent.*;

// try-with-resources + NIO.2
void read(Path path) throws Exception {
    try (InputStream in = Files.newInputStream(path)) { /* 自动 close */ }
}

import java.util.Objects;
import java.nio.file.Paths;

String cmd = "go";
switch (cmd) {
    case "go": break;
    default: break;
}
List<String> a = new ArrayList<>();      // diamond
Path p = Paths.get("a", "b", "c.txt");
Objects.requireNonNull(p, "p");

// ForkJoinPool.commonPool() 在 1.7 引入
import java.util.concurrent.RecursiveTask;

class Sum extends RecursiveTask<Integer> { /* compute/split 省略 */ }

import java.util.*;
import java.util.stream.Collectors;

List<String> names = Arrays.asList("a", "bb", "ccc");
List<String> longOnes = names.stream()
    .filter(s -> s.length() > 1)
    .map(String::toUpperCase)
    .collect(Collectors.toList());

interface Greeter {
    default void hello() { System.out.println("Hi"); }
}

import java.time.*;
import java.time.format.DateTimeFormatter;
LocalDate d = LocalDate.now();
ZonedDateTime z = ZonedDateTime.now(ZoneId.of("Asia/Shanghai"));

import java.util.Arrays;
int sum = Arrays.asList(1, 2, 3).stream().mapToInt(i -> i).sum();

module com.example.app {
    requires java.base;
    exports com.example.api;
}

List<String> list = List.of("a", "b");

interface Util {
    private static int add(int a, int b) { return a + b; }
    default int twice(int x) { return add(x, x); }
}

var list = new ArrayList<String>();
for (var s : list) { }

"  \n".isBlank();
"line1\nline2".lines().forEach(System.out::println);

import java.net.URI;
import java.net.http.*;
import java.time.Duration;

HttpClient c = HttpClient.newBuilder()
    .connectTimeout(Duration.ofSeconds(5))
    .build();
HttpRequest r = HttpRequest.newBuilder(URI.create("https://httpbin.org/get"))
    .GET()
    .build();
// HttpResponse<String> res = c.send(r, HttpResponse.BodyHandlers.ofString());

java Hello.java

import java.lang.constant.ClassDesc;
import java.lang.constant.MethodTypeDesc;

ClassDesc str = ClassDesc.of("java.lang.String");
MethodTypeDesc mt = MethodTypeDesc.ofDescriptor("()V");
System.out.println(str.displayName() + " / " + mt.descriptorString());

import java.text.NumberFormat;
import java.util.Locale;

NumberFormat f = NumberFormat.getCompactNumberInstance(
        Locale.CHINA, NumberFormat.Style.SHORT);
System.out.println(f.format(12_000_000)); // 显示形式依 Locale

// 预览：在 12 中需 --enable-preview；演示形态，省略 month/year 等上下文
// int days = switch (m) { case 1,3,5 -> 31; default -> 30; };

// 与 Java 5+ 同形态；JDK 13 中底层实现为新 Socket 实现
try (java.net.Socket s = new java.net.Socket()) {
    s.connect(new java.net.InetSocketAddress("openjdk.org", 443), 5_000);
}

// javac 13: --enable-preview
String html = """
    <html>
      <body>Hi</body>
    </html>
    """;

// Java 14+ 默认即可编译
int n = 2;
String s = switch (n) {
    case 1, 2 -> "a or b";
    default -> "other";
};

Object obj = "x";
if (obj instanceof String str) { System.out.println(str.length()); } // 预览
// record Point(int x, int y) { }  // 预览，宜单独文件/类

String t = """
    two lines
    ok
    """;  // 二轮 文本 预览

String json = """
    {
      "a": 1
    }
    """;
System.out.println(json.stripIndent());

// 在 16/17 前通常需按发行说明启用 preview
// public abstract sealed class Shape permits Circle, Rect { }

public class R16 {
    public record User(String name, int age) { }

    public static void main(String[] args) {
        var u = new User("Ann", 20);
        System.out.println(u.name());
        Object o = u;
        if (o instanceof User x) {
            System.out.println(x.age());
        }
    }
}

sealed interface Shape permits Circle, Rect { }
final class Circle implements Shape { }
final class Rect implements Shape { }

import java.util.random.RandomGenerator;

RandomGenerator g = RandomGenerator.getDefault();
int n = g.nextInt(0, 10);

// static String pick(Object o) { return switch (o) { case null -> "n"; case String s -> s; default -> o.toString(); }; }

import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;

Charset d = Charset.defaultCharset();
assert StandardCharsets.UTF_8.equals(d)
        || true; // 少数配置仍可能非 UTF-8，以运行环境为准
System.out.println(d);

# 在含 index.html 的目录执行（需 JDK 18+ 且安装含 jwebserver 的 build）
# jwebserver -p 8080

// javac 19: --enable-preview --release 19
class Rp19 {
    record Rectangle(int x, int y, int w, int h) { }

    int area(Object o) {
        if (o instanceof Rectangle(int x, int y, int w, int h)) {
            return w * h;
        }
        return 0;
    }
}

import java.net.InetAddress;

InetAddress a = InetAddress.getByName("192.0.2.1");
System.out.println(a.getHostAddress());

import jdk.jfr.FlightRecorder;
import jdk.jfr.Recording;

if (FlightRecorder.isAvailable()) {
    try (Recording r = new Recording()) {
        r.setName("demo");
        r.setToDisk(true);
        r.start();
        Thread.sleep(1);
        r.stop();
    }
}

try (var ex = java.util.concurrent.Executors.newVirtualThreadPerTaskExecutor()) {
    ex.submit(() -> { /* 大量 IO */ });
}

// 模式 switch + 记录 + 密封：在 21 为 终态（对 前序 版 的 预览 形态 的 定稿），默认 无需 --enable-preview
public class Pets {
    sealed interface Pet permits Cat, Dog { }
    record Cat(String n) implements Pet { }
    record Dog(String n) implements Pet { }

    static String label(Pet p) {
        return switch (p) {
            case Cat c  -> "cat " + c.n();
            case Dog d  -> "dog " + d.n();
        };
    }
}

import java.util.*;

SequencedCollection<String> sc = new LinkedList<>();
sc.addLast("a");
sc.addFirst("b");
// sc.getFirst();

// void main() { }  // 在 25 前多为 preview；见 JEP 445/512 轨迹

import java.lang.foreign.*;
import java.lang.invoke.VarHandle;

ValueLayout.OfInt L = ValueLayout.JAVA_INT;
VarHandle h = L.varHandle();
try (Arena arena = Arena.ofConfined()) {
    MemorySegment seg = arena.allocate(4, 4);
    h.set(seg, 0L, 22);
    System.out.println((int) h.get(seg, 0L));
}

/**
 * # 标题示例
 * - 列表 与 **加粗**（具体支持度以本 JDK 的 `javadoc` 帮助为准）
 */
public class ApiDoced23 {
    /** 单句摘要 */
    public void run() { }
}

# 对上一文件生成 文档 时 启用 Markdown（以本机 javadoc 支持为准，例如 23+）
# javadoc -d out ApiDoced23.java

import java.util.List;
import java.util.stream.Gatherers;
import java.util.stream.Stream;

// 将元素按 固定 窗长 分块
List<List<Integer>> windows = Stream.of(1, 2, 3, 4, 5, 6, 7, 8)
        .gather(Gatherers.windowFixed(3))
        .toList();
System.out.println(windows);

import module java.base;
import module java.sql;
// import module com.example.mylib;  // 你方模块需在 module-info 中 requires 对应模块

import module java.base;

// 本文件在「未命名模块」、类路径上时，仍可使用对 JDK 的 import module
class PrintArgs {
    public static void main(String[] args) {
        var list = new java.util.ArrayList<String>();
        for (var a : args) {
            list.add(a);
        }
        System.out.println(String.join(", ", list));
    }
}

javac --release 25 PrintArgs.java
java PrintArgs a b c

import module java.desktop;
import module java.base;
import java.awt.Label;   // 明确要 AWT 的 Label
import java.util.List;  // 明确要 java.util 的 List

// ... 在方法中使用 Label、List

import module java.base;

void main() {
    IO.println("Hello, Java 25");
}

class Hello {
    void main() {
        IO.println("Hi");
    }
}

import module java.base;

class Launcher {
    public static void main(String[] args) {
        IO.println("args=" + java.util.List.of(args));
    }

    void main() {
        IO.println("只有删除上面的 static main 时才会用实例 main");
    }
}

import module java.base;

String greet(String name) {
    return "Hello, " + name;
}

void main() {
    IO.println(greet("Java"));
}

java Hello.java
# 或：javac --release 25 Hello.java  &&  java Hello   # 具体可运行类名以 javac 输出为准

class Base {
    Base(int v) { }
}

class Sub extends Base {
    private final int v;

    Sub(int raw) {
        int x = Math.max(0, raw);
        super(x);
        this.v = x;
    }
}

import java.util.Objects;

class Base2 {
    Base2(int len) { }
}

class Sub2 extends Base2 {
    Sub2(String s) {
        String t = Objects.requireNonNull(s, "s");
        super(t.length());
    }
}

import java.util.concurrent.ScopedValue;

public class ScopedDemo {
    private static final ScopedValue<String> USER = ScopedValue.newInstance();

    public static void main(String[] args) {
        ScopedValue.where(USER, "alice").run(() -> {
            System.out.println(USER.get());
            m();
        });
    }

    static void m() {
        System.out.println("in m: " + USER.get());
    }
}

import javax.crypto.KDF;
import javax.crypto.SecretKey;
import javax.crypto.spec.HKDFParameterSpec;
import java.nio.charset.StandardCharsets;
import java.security.SecureRandom;
import java.security.spec.AlgorithmParameterSpec;
import java.util.Base64;

public class KdfDemo {
    public static void main(String[] args) throws Exception {
        byte[] ikm = new byte[32];
        new SecureRandom().nextBytes(ikm);
        byte[] salt = new byte[16];
        new SecureRandom().nextBytes(salt);

        KDF hkdf = KDF.getInstance("HKDF-SHA256");

        byte[] encInfo = "app|v1|enc".getBytes(StandardCharsets.UTF_8);
        AlgorithmParameterSpec encSpec = HKDFParameterSpec.ofExtract()
                .addIKM(ikm)
                .addSalt(salt)
                .thenExpand(encInfo, 32);
        SecretKey aes = hkdf.deriveKey("AES", encSpec);

        byte[] macInfo = "app|v1|mac".getBytes(StandardCharsets.UTF_8);
        AlgorithmParameterSpec macSpec = HKDFParameterSpec.ofExtract()
                .addIKM(ikm)
                .addSalt(salt)
                .thenExpand(macInfo, 32);
        byte[] macKey = hkdf.deriveData(macSpec);

        System.out.println("AES: " + Base64.getEncoder().encodeToString(aes.getEncoded()));
        System.out.println("MAC: " + Base64.getEncoder().encodeToString(macKey));
    }
}

javac --release 25 KdfDemo.java
java KdfDemo

javac --release 25 --enable-preview   YourFile.java
java  --enable-preview   YourFile
# 或源码直跑：java --enable-preview YourFile.java

// javac --release 25 --enable-preview StablePreview.java
// java  --enable-preview StablePreview

import java.lang.StableValue;
import java.util.logging.Logger;

public class StablePreview {
    private static final StableValue<Logger> LOG = StableValue.of();

    private static Logger logger() {
        return LOG.orElseSet(() -> Logger.getLogger(StablePreview.class.getName()));
    }

    public static void main(String[] args) {
        logger().info("ok");
    }
}

// 保存为 MapStatus.java
// 编译/运行需：javac --release 25 --enable-preview MapStatus.java
//              java  --enable-preview MapStatus
public class MapStatus {
    public static void main(String[] args) {
        System.out.println(mapStatus(99));
    }

    static String mapStatus(int code) {
        return switch (code) {
            case 0 -> "ok";
            case 1 -> "warn";
            case 2 -> "err";
            case int i -> "other: " + i;   // 绑定“其余 int 值”
        };
    }
}

// StructConcDemo.java
import java.util.concurrent.StructuredTaskScope;

public class StructConcDemo {
    public static void main(String[] args) throws Exception {
        try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
            var a = scope.fork(() -> "x");
            var b = scope.fork(() -> 1);
            scope.join();
            scope.throwIfFailed();
            System.out.println(a.get() + b.get());
        }
    }
}

javac --release 25 StructConcDemo.java
java  StructConcDemo

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.time.Duration;

// 需 Java 26+ 且 本 版本 的 HttpClient 暴露 Version.HTTP_3
HttpClient client = HttpClient.newBuilder()
        .version(HttpClient.Version.HTTP_3)
        .connectTimeout(Duration.ofSeconds(10))
        .build();

HttpRequest request = HttpRequest.newBuilder(URI.create("https://openjdk.org/"))
        .GET()
        .build();

// HttpResponse<String> res = client.send(request, HttpResponse.BodyHandlers.ofString());
// System.out.println(res.version());

java -version

javac --release 17 -d out src/com/example/App.java

<properties>
  <maven.compiler.release>17</maven.compiler.release>
</properties>