Java 语法速览

发表于 更新于 分类于 ，

1. Hello World

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

2. 变量与数据类型

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public class Main {
    public static void main(String[] args) {
        int a = 10;
        double b = 3.14;
        char c = 'A';
        boolean d = true;
        String s = "Java";
        System.out.println(a + " " + b + " " + c + " " + d + " " + s);
    }
}

3. 条件语句（if / else / switch）

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public class Main {
    public static void main(String[] args) {
        int x = 5;
        if (x > 0) System.out.println("Positive");
        else if (x < 0) System.out.println("Negative");
        else System.out.println("Zero");

        switch (x) {
            case 1: System.out.println("One"); break;
            case 5: System.out.println("Five"); break;
            default: System.out.println("Other");
        }
    }
}

4. 循环（for / while / do-while）

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public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < 3; i++) System.out.println("For: " + i);
        int j = 0;
        while (j < 3) System.out.println("While: " + j++);
        int k = 0;
        do { System.out.println("DoWhile: " + k++); } while (k < 3);
    }
}

5. 方法定义与调用

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public class Main {
    static int add(int a, int b) {
        return a + b;
    }
    public static void main(String[] args) {
        System.out.println(add(3, 4));
    }
}

6. 数组与字符串

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public class Main {
    public static void main(String[] args) {
        int[] nums = {1, 2, 3};
        for (int n : nums) System.out.print(n + " ");
        
        String s = "Hello";
        System.out.println("\n" + s.toUpperCase());
    }
}

7. 类与对象

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class Person {
    String name;
    int age;
    void greet() {
        System.out.println("Hi, I'm " + name);
    }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person();
        p.name = "Alice";
        p.age = 25;
        p.greet();
    }
}

8. 构造函数与 this 关键字

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class Person {
    String name;
    int age;
    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    void show() {
        System.out.println(name + ", " + age);
    }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Bob", 30);
        p.show();
    }
}

9. 继承与方法重写（@Override）

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class Animal {
    void speak() {
        System.out.println("Animal speaks");
    }
}

class Dog extends Animal {
    @Override
    void speak() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Dog();
        a.speak();
    }
}

10. 接口与多态

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interface Drawable {
    void draw();
}

class Circle implements Drawable {
    public void draw() {
        System.out.println("Drawing Circle");
    }
}

public class Main {
    public static void main(String[] args) {
        Drawable d = new Circle();
        d.draw();
    }
}

11. 异常处理（try-catch-finally）

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public class Main {
    public static void main(String[] args) {
        try {
            int x = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Error: " + e.getMessage());
        } finally {
            System.out.println("Done");
        }
    }
}

12. 静态方法和变量

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class Utils {
    static int count = 0;
    static void greet() {
        System.out.println("Hello from static method");
    }
}

public class Main {
    public static void main(String[] args) {
        Utils.greet();
        System.out.println(Utils.count);
    }
}

13. 枚举（enum）

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enum Color {
    RED, GREEN, BLUE
}

public class Main {
    public static void main(String[] args) {
        Color c = Color.GREEN;
        switch (c) {
            case RED -> System.out.println("Red");
            case GREEN -> System.out.println("Green");
            case BLUE -> System.out.println("Blue");
        }
    }
}

14. 集合框架（List / Set / Map）

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import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("A", "B", "C");
        Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 2));
        Map<String, Integer> map = Map.of("a", 1, "b", 2);

        list.forEach(System.out::println);
        set.forEach(System.out::println);
        map.forEach((k, v) -> System.out.println(k + ": " + v));
    }
}

15. Lambda 表达式与 Stream API（Java 8+）

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

public class Main {
    public static void main(String[] args) {
        List<Integer> nums = Arrays.asList(1, 2, 3, 4, 5);
        nums.stream().filter(n -> n % 2 == 0).forEach(System.out::println);
    }
}

16. 多线程基础：Thread 与 Runnable

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public class Main {
    public static void main(String[] args) {
        Thread t = new Thread(() -> System.out.println("Hello from thread!"));
        t.start();
    }
}

17. 泛型（类、方法、限定通配符）

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class Box<T> {
    T value;
    Box(T value) { this.value = value; }
    T get() { return value; }
}

public class Main {
    public static void main(String[] args) {
        Box<String> b = new Box<>("Hello");
        System.out.println(b.get());
    }
}

泛型方法示例：

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public class Main {
    public static <T> void printArray(T[] array) {
        for (T t : array) System.out.println(t);
    }

    public static void main(String[] args) {
        printArray(new Integer[]{1, 2, 3});
        printArray(new String[]{"a", "b", "c"});
    }
}

18. 内部类与匿名类

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class Outer {
    int outerVal = 10;

    class Inner {
        void show() {
            System.out.println("Inner: " + outerVal);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Outer.Inner inner = new Outer().new Inner();
        inner.show();
    }
}

匿名类：

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public class Main {
    interface Hello {
        void say();
    }

    public static void main(String[] args) {
        Hello h = new Hello() {
            public void say() {
                System.out.println("Hello from anonymous class");
            }
        };
        h.say();
    }
}

19. 注解（Annotation）

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@Retention(RetentionPolicy.RUNTIME)
@interface Info {
    String author();
    int version();
}

@Info(author = "Yao", version = 1)
public class Main {
    public static void main(String[] args) {
        Info info = Main.class.getAnnotation(Info.class);
        System.out.println(info.author() + " v" + info.version());
    }
}

20. 反射基础

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import java.lang.reflect.Method;

public class Main {
    public static void main(String[] args) throws Exception {
        Class<?> cls = Class.forName("java.lang.String");
        for (Method m : cls.getDeclaredMethods()) {
            if (m.getName().equals("length"))
                System.out.println("Found: " + m);
        }
    }
}

21. 文件读写（IO）

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import java.io.*;

public class Main {
    public static void main(String[] args) throws IOException {
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("test.txt"))) {
            writer.write("Hello File!");
        }
        try (BufferedReader reader = new BufferedReader(new FileReader("test.txt"))) {
            System.out.println(reader.readLine());
        }
    }
}

22. NIO 文件读取（Java 7+）

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import java.nio.file.*;

public class Main {
    public static void main(String[] args) throws Exception {
        String content = Files.readString(Path.of("test.txt"));
        System.out.println("Read: " + content);
    }
}

23. 枚举增强用法：带方法和属性

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enum Level {
    LOW(1), MEDIUM(2), HIGH(3);
    int val;
    Level(int v) { this.val = v; }
    public int getVal() { return val; }
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Level.HIGH.getVal());
    }
}

24. Optional（避免 NullPointerException）

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import java.util.Optional;

public class Main {
    public static void main(String[] args) {
        Optional<String> maybe = Optional.of("Hello");
        System.out.println(maybe.map(String::toUpperCase).orElse("Empty"));
    }
}

25. synchronized 关键字：线程安全同步

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public class Main {
    private static int count = 0;

    public static synchronized void increment() {
        count++;
    }

    public static void main(String[] args) throws InterruptedException {
        Thread t1 = new Thread(() -> { for (int i = 0; i < 1000; i++) increment(); });
        Thread t2 = new Thread(() -> { for (int i = 0; i < 1000; i++) increment(); });
        t1.start(); t2.start(); t1.join(); t2.join();
        System.out.println(count); // 应该是2000
    }
}

26. 线程池 ExecutorService

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import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {
        ExecutorService executor = Executors.newFixedThreadPool(2);
        Future<Integer> future = executor.submit(() -> 1 + 2);
        System.out.println("Result: " + future.get());
        executor.shutdown();
    }
}

27. volatile 保证变量可见性

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public class Main {
    private static volatile boolean running = true;

    public static void main(String[] args) throws Exception {
        Thread t = new Thread(() -> {
            while (running);
            System.out.println("Stopped");
        });
        t.start();
        Thread.sleep(1000);
        running = false;
    }
}

28. ReentrantLock 可重入锁

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import java.util.concurrent.locks.ReentrantLock;

public class Main {
    private static final ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) {
        lock.lock();
        try {
            System.out.println("Locked section");
        } finally {
            lock.unlock();
        }
    }
}

29. Stream API：map/filter/collect

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

public class Main {
    public static void main(String[] args) {
        List<String> list = List.of("apple", "banana", "orange");
        List<String> upper = list.stream()
            .filter(s -> s.startsWith("a"))
            .map(String::toUpperCase)
            .collect(Collectors.toList());
        System.out.println(upper); // [APPLE]
    }
}

30. record（Java 14+）：简化数据类

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record Person(String name, int age) {}

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Alice", 30);
        System.out.println(p.name() + ", " + p.age());
    }
}

31. sealed class（Java 15+）

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sealed interface Shape permits Circle, Square {}

final class Circle implements Shape {}
final class Square implements Shape {}

public class Main {
    public static void main(String[] args) {
        Shape s = new Circle();
        if (s instanceof Circle) System.out.println("It's a Circle");
    }
}

32. 简单 Java GUI 示例（Swing）

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import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Hello GUI");
        JButton button = new JButton("Click Me");
        button.addActionListener(e -> JOptionPane.showMessageDialog(frame, "Hello"));
        frame.add(button);
        frame.setSize(200, 100);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

33. Socket 网络通信（客户端）

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import java.io.*;
import java.net.*;

public class Main {
    public static void main(String[] args) throws IOException {
        Socket socket = new Socket("example.com", 80);
        PrintWriter out = new PrintWriter(socket.getOutputStream());
        out.print("GET / HTTP/1.1\r\nHost: example.com\r\n\r\n");
        out.flush();

        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        System.out.println(in.readLine());

        socket.close();
    }
}

34. JDBC 简单访问（以 SQLite 为例）

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import java.sql.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Class.forName("org.sqlite.JDBC");
        Connection conn = DriverManager.getConnection("jdbc:sqlite:test.db");
        Statement stmt = conn.createStatement();
        stmt.executeUpdate("CREATE TABLE IF NOT EXISTS users (name TEXT)");
        stmt.executeUpdate("INSERT INTO users VALUES ('Alice')");
        ResultSet rs = stmt.executeQuery("SELECT * FROM users");
        while (rs.next()) {
            System.out.println(rs.getString("name"));
        }
        conn.close();
    }
}

35. Java 模块系统（Java 9+）

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// 文件：module-info.java
module com.example.hello {
    exports com.example.hello;
}
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// 文件：com/example/hello/Hello.java
package com.example.hello;

public class Hello {
    public static void say() {
        System.out.println("Hello from module!");
    }
}
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// 文件：Main.java
import com.example.hello.Hello;

public class Main {
    public static void main(String[] args) {
        Hello.say();
    }
}

36. Lambda 表达式 与 函数式接口

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import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, Integer> strLength = s -> s.length();
        System.out.println(strLength.apply("Hello")); // 输出 5
    }
}

37. CompletableFuture 异步编程

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import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) throws Exception {
        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello")
                .thenApply(s -> s + " World");
        System.out.println(future.get()); // 输出 "Hello World"
    }
}

38. JUnit5 单元测试基础

确保使用 JDK + Maven 或 IDE 支持，添加依赖后可运行：

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// 文件名: Calculator.java
public class Calculator {
    public int add(int a, int b) {
        return a + b;
    }
}
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// 文件名: CalculatorTest.java
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

public class CalculatorTest {
    @Test
    void testAdd() {
        Calculator calc = new Calculator();
        assertEquals(5, calc.add(2, 3));
    }
}

（需要 JUnit 5 依赖，Maven 示例可提供）

39. 使用 Gson 解析和生成 JSON

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import com.google.gson.*;

class Person {
    String name;
    int age;
}

public class Main {
    public static void main(String[] args) {
        String json = "{\"name\":\"Alice\",\"age\":30}";
        Gson gson = new Gson();
        Person p = gson.fromJson(json, Person.class);
        System.out.println(p.name + " is " + p.age);
    }
}

（需添加 Gson 依赖：com.google.code.gson:gson:2.10.1）

40. 使用 Maven 构建项目（示例 pom.xml）

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<!-- 文件名: pom.xml -->
<project xmlns="http://maven.apache.org/POM/4.0.0" ...>
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>demo</artifactId>
  <version>1.0</version>
  <dependencies>
    <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.10.1</version>
    </dependency>
  </dependencies>
</project>

41. 使用 Gradle 构建项目（示例 build.gradle）

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plugins {
    id 'java'
}

repositories {
    mavenCentral()
}

dependencies {
    implementation 'com.google.code.gson:gson:2.10.1'
    testImplementation 'org.junit.jupiter:junit-jupiter:5.10.0'
}

test {
    useJUnitPlatform()
}

42. Java Servlet 示例（Java Web）

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import jakarta.servlet.*;
import jakarta.servlet.http.*;
import java.io.*;

public class HelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        PrintWriter out = resp.getWriter();
        out.println("Hello from Servlet!");
    }
}

（需部署到如 Tomcat、Jetty 的 Servlet 容器）

43. 注解（Annotation）与反射使用

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import java.lang.annotation.*;
import java.lang.reflect.Method;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface MyAnnotation {
    String value();
}

public class Main {
    @MyAnnotation("HelloAnnotation")
    public void sayHello() {
        System.out.println("Hello");
    }

    public static void main(String[] args) throws Exception {
        Method method = Main.class.getMethod("sayHello");
        if (method.isAnnotationPresent(MyAnnotation.class)) {
            MyAnnotation ann = method.getAnnotation(MyAnnotation.class);
            System.out.println("Annotation value: " + ann.value());
        }
        new Main().sayHello();
    }
}

44. 反射动态创建类与调用方法

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public class Main {
    public static void greet(String name) {
        System.out.println("Hello, " + name);
    }

    public static void main(String[] args) throws Exception {
        Class<?> clazz = Class.forName("Main");
        Method method = clazz.getMethod("greet", String.class);
        method.invoke(null, "World");
    }
}

45. 多线程基础：实现 Runnable 接口

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public class Main implements Runnable {
    public void run() {
        System.out.println("Thread running: " + Thread.currentThread().getName());
    }

    public static void main(String[] args) {
        Thread t = new Thread(new Main());
        t.start();
    }
}

46. 多线程：使用 synchronized 同步

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public class Counter {
    private int count = 0;

    public synchronized void increment() {
        count++;
    }

    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Thread t1 = new Thread(() -> { for (int i = 0; i < 1000; i++) counter.increment(); });
        Thread t2 = new Thread(() -> { for (int i = 0; i < 1000; i++) counter.increment(); });
        t1.start(); t2.start();
        t1.join(); t2.join();
        System.out.println("Count: " + counter.count); // 2000
    }
}

47. 使用 ReentrantLock 进行加锁

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import java.util.concurrent.locks.ReentrantLock;

public class Main {
    private final ReentrantLock lock = new ReentrantLock();
    private int count = 0;

    public void increment() {
        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Main main = new Main();
        Thread t1 = new Thread(() -> { for (int i = 0; i < 1000; i++) main.increment(); });
        Thread t2 = new Thread(() -> { for (int i = 0; i < 1000; i++) main.increment(); });
        t1.start(); t2.start(); t1.join(); t2.join();
        System.out.println("Count: " + main.count); // 2000
    }
}

48. volatile 保证变量可见性

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public class Main {
    private volatile boolean running = true;

    public void run() {
        while (running) {
            // 循环等待
        }
        System.out.println("Stopped.");
    }

    public void stop() {
        running = false;
    }

    public static void main(String[] args) throws InterruptedException {
        Main m = new Main();
        Thread t = new Thread(m::run);
        t.start();
        Thread.sleep(1000);
        m.stop();
    }
}

49. Socket 编程：简单服务器与客户端

Server.java

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import java.io.*;
import java.net.*;

public class Server {
    public static void main(String[] args) throws IOException {
        ServerSocket server = new ServerSocket(6666);
        Socket socket = server.accept();
        BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        System.out.println("Client says: " + reader.readLine());
        server.close();
    }
}

Client.java

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import java.io.*;
import java.net.*;

public class Client {
    public static void main(String[] args) throws IOException {
        Socket socket = new Socket("localhost", 6666);
        PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);
        writer.println("Hello from client!");
        socket.close();
    }
}

50. 枚举类型的高级用法

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enum Operation {
    PLUS { double apply(double x, double y) { return x + y; } },
    MINUS { double apply(double x, double y) { return x - y; } };

    abstract double apply(double x, double y);
}

public class Main {
    public static void main(String[] args) {
        System.out.println(Operation.PLUS.apply(2, 3));  // 5.0
        System.out.println(Operation.MINUS.apply(5, 2)); // 3.0
    }
}

继续为你整理 Java 更高级和实用的语法特性（第 51 ～ 60 部分），每个示例都可直接运行。

51. Java Stream 高级操作：groupingBy + averaging

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

public class Main {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Tom", "Jerry", "Tom", "Spike", "Jerry");

        Map<String, Long> counts = names.stream()
            .collect(Collectors.groupingBy(name -> name, Collectors.counting()));

        System.out.println(counts); // {Jerry=2, Tom=2, Spike=1}
    }
}

52. try-with-resources 自动释放资源

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import java.io.*;

public class Main {
    public static void main(String[] args) {
        try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {
            System.out.println(br.readLine());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

test.txt 文件需在项目根目录。适用于任何 AutoCloseable 类型。

53. record：简化 Java Bean（Java 14+）

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

public class Main {
    public static void main(String[] args) {
        User u = new User("Tom", 30);
        System.out.println(u.name() + ", " + u.age());
    }
}

54. Java Lambda 表达式与函数式接口

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import java.util.function.Function;

public class Main {
    public static void main(String[] args) {
        Function<String, Integer> parseLength = str -> str.length();
        System.out.println(parseLength.apply("Hello")); // 5
    }
}

55. Optional 使用：避免 NullPointerException

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import java.util.Optional;

public class Main {
    public static void main(String[] args) {
        Optional<String> name = Optional.ofNullable(null);
        String result = name.orElse("Default");
        System.out.println(result); // Default
    }
}

56. 接口的默认方法（Java 8+）

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interface Greeter {
    default void greet() {
        System.out.println("Hello from default method!");
    }
}

public class Main implements Greeter {
    public static void main(String[] args) {
        new Main().greet();
    }
}

57. 文件读写（Java NIO）

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import java.nio.file.*;
import java.io.IOException;

public class Main {
    public static void main(String[] args) throws IOException {
        Path path = Paths.get("output.txt");
        Files.write(path, "Hello NIO".getBytes());

        String content = Files.readString(path);
        System.out.println(content);
    }
}

58. 使用 ThreadPoolExecutor 线程池

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import java.util.concurrent.*;

public class Main {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(2);
        executor.submit(() -> System.out.println("Task 1"));
        executor.submit(() -> System.out.println("Task 2"));
        executor.shutdown();
    }
}

59. 动态代理（Proxy）

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import java.lang.reflect.*;

interface Hello {
    void say();
}

public class Main {
    public static void main(String[] args) {
        Hello hello = (Hello) Proxy.newProxyInstance(
            Hello.class.getClassLoader(),
            new Class[]{Hello.class},
            (proxy, method, args1) -> {
                System.out.println("Before method");
                return null;
            }
        );
        hello.say(); // Before method
    }
}

60. 简单 GUI（Swing 示例）

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import javax.swing.*;

public class Main {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Hello GUI");
        JButton button = new JButton("Click Me");
        button.addActionListener(e -> JOptionPane.showMessageDialog(null, "Button clicked!"));
        frame.add(button);
        frame.setSize(300, 200);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}