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

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        System.out.println("Input q to exit.");
        try (Scanner input = new Scanner(System.in)) {
            while (true) {
                String line = input.nextLine();
                if (line.equalsIgnoreCase("q")) {
                    break;
                }
                System.out.println("echo:" + line);
            }
        }
        System.out.println("exited.");
    }
}

import java.net.ServerSocket;
import java.net.Socket;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.IOException;

public class Main {

    public static void main(String[] args) {
        new Thread(Main::serverSocket).start();

        System.out.println("Please input:");
        try (var s = new Socket("127.0.0.1", 12345);
                var out = new PrintWriter(s.getOutputStream(), true);
                var in = new BufferedReader(new InputStreamReader(s.getInputStream()));
                var stdIn = new BufferedReader(new InputStreamReader(System.in))) {
            String userInput;
            while ((userInput = stdIn.readLine()) != null) {
                out.println(userInput);
                System.out.println("Echo: " + in.readLine());
            }
        } catch (IOException e) {
            System.err.println(e);
        }
    }

    public static void serverSocket() {
        try (var ss = new ServerSocket(12345); var s = ss.accept();
                var out = new PrintWriter(s.getOutputStream(), true);
                var in = new BufferedReader(new InputStreamReader(s.getInputStream()))) {
            String inputLine;
            while ((inputLine = in.readLine()) != null) {
                out.println(inputLine);
            }
        } catch (IOException e) {
            System.err.println(e);
        }
    }
}

public static void main(String[] args) {
  String url = "jdbc:mysql://127.0.0.1:3306/db-name";
  String user = "root", password = "1";
  try (Connection conn = DriverManager.getConnection(url, user, password);
        Statement sta = conn.createStatement()) {
    ResultSet rs = sta.executeQuery("select 'Hello, World!'");
    rs.next();
    System.out.println(rs.getObject(1));
  }
}

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.SubmissionPublisher;
import java.util.stream.IntStream;

public class Main {

    public static void main(String[] args) {
        CompletableFuture<Void> subTask;
        try (var pub = new SubmissionPublisher<Integer>()) {
            // 该方法内部订阅了ConsumerSubscriber(函数式接口入参)
            subTask = pub.consume(System.out::println);
            /*  Or 自定义订阅器(onNext方法可执行传入的函数式接口)
            pub.subscribe(new Flow.Subscriber<Integer>() {
                @Override
                public void onSubscribe(Flow.Subscription subscription) {
                    subscription.request(10); // 此处决定消费量
                }
                @Override public void onNext(Integer item) {
                    System.out.println(item);
                    // consumer.accept(item);
                }
                @Override public void onComplete() { }
                @Override public void onError(Throwable throwable) { }
            });  */
            IntStream.range(1, 6).forEach(pub::submit);
        }
        // 因pub.subscribe方法无返回值，故可用sleep方法来阻塞主线程等待执行结果。
        // try{ TimeUnit.SECONDS.sleep(3); }catch(InterruptedException ex){ }
        if (subTask != null) {
            try {
                subTask.get();
            } catch (InterruptedException | ExecutionException e) {
                System.err.println(e);
            }
        }
    }
}

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class Hi {

    public static void main(String[] args) {
        SpringApplication.run(Main.class, args);
    }
}

@RestController
class HelloWorld {

    @RequestMapping("/")
    public String helloworld() {
        return "Hello, World!";
    }
}