Of course! "Java sendmessage" is a broad topic because "send message" can mean many different things depending on the context. Here’s a comprehensive guide covering the most common interpretations, from simple console output to advanced messaging systems.

I'll break it down into the following scenarios:

1. The Basics: Printing to the Console
2. GUI Applications: Displaying a Message in a Window
3. Client-Server Communication: Sending a Message over a Network (Socket)
4. Modern Asynchronous Messaging: Using a Message Queue (RabbitMQ example)
5. Sending Notifications: Sending an Email (using JavaMail)

The Basics: Printing to the Console

This is the most fundamental way to "send a message" in Java. It's used for logging, debugging, and simple command-line interaction.

Code Example:

public class ConsoleMessage {
    public static void main(String[] args) {
        // A simple string message
        System.out.println("Hello, this is a message from Java!");
        // A message with a variable
        String userName = "Alice";
        System.out.println("Welcome, " + userName + "!");
        // Using formatted strings (recommended for complex messages)
        int userScore = 100;
        System.out.printf("User %s has a score of %d.%n", userName, userScore);
    }
}

• System.out.println(): Prints a message to the console and adds a new line at the end.
• System.out.print(): Prints a message without adding a new line.
• System.out.printf(): Prints a formatted string, similar to C's printf.

GUI Applications: Displaying a Message in a Window

If you're building a graphical user interface (GUI), "send message" usually means showing a dialog box to the user. The standard library for this is Swing.

Code Example (using Swing):

import javax.swing.JOptionPane;
public class GuiMessage {
    public static void main(String[] args) {
        // Display a simple information message dialog
        JOptionPane.showMessageDialog(null, "This is an informational message.");
        // Display a warning message dialog
        JOptionPane.showMessageDialog(null, "Warning: File not found!", "Warning", JOptionPane.WARNING_MESSAGE);
        // Display an error message dialog
        JOptionPane.showMessageDialog(null, "A critical error has occurred.", "Error", JOptionPane.ERROR_MESSAGE);
        // Display a confirmation dialog (Yes/No/Cancel)
        int option = JOptionPane.showConfirmDialog(null, "Do you want to save your changes?");
        if (option == JOptionPane.YES_OPTION) {
            System.out.println("User chose YES.");
        } else if (option == JOptionPane.NO_OPTION) {
            System.out.println("User chose NO.");
        } else {
            System.out.println("User chose CANCEL.");
        }
    }
}

• JOptionPane is a utility class in Swing that provides pre-configured dialog boxes for simple interactions.
• The null argument as the first parameter centers the dialog on the screen.

Client-Server Communication: Sending a Message over a Network

This is a common pattern for applications where two or more systems need to communicate. We'll use Java Sockets, which provide a low-level way to send data over a TCP/IP network.

Server Code (Listens for messages and prints them):

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;
public class SimpleServer {
    public static void main(String[] args) {
        int port = 6789;
        try (ServerSocket serverSocket = new ServerSocket(port)) {
            System.out.println("Server is listening on port " + port);
            // The server waits here for a client to connect
            Socket clientSocket = serverSocket.accept();
            System.out.println("Client connected: " + clientSocket.getInetAddress());
            // Set up to read data from the client
            BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            String messageFromClient = in.readLine();
            System.out.println("Message received from client: " + messageFromClient);
        } catch (IOException e) {
            System.out.println("Server exception: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

Client Code (Connects to the server and sends a message):

import java.io.IOException;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;
public class SimpleClient {
    public static void main(String[] args) {
        String hostName = "localhost"; // or the server's IP address
        int port = 6789;
        try (Socket socket = new Socket(hostName, port);
             PrintWriter out = new PrintWriter(socket.getOutputStream(), true)) {
            System.out.println("Connected to server. Sending message...");
            // The actual "send message" part
            out.println("Hello from the Java client!");
        } catch (UnknownHostException e) {
            System.err.println("Don't know about host " + hostName);
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Couldn't get I/O for the connection to " +
                    hostName);
            System.exit(1);
        }
    }
}

How to run it:

1. Run the SimpleServer class first. It will wait.
2. Run the SimpleClient class. The server will receive the message "Hello from the Java client!".

Modern Asynchronous Messaging: Using a Message Queue

For large-scale, decoupled, and reliable applications, direct sockets are often replaced by a Message Queue. The producer application sends a message to a queue, and one or more consumer applications process it. This is highly scalable and resilient.

We'll use RabbitMQ as a popular example.

Prerequisites:

1. Install RabbitMQ on your machine or use a cloud service.
2. Add the RabbitMQ Java client library to your project (e.g., in Maven, add this to your pom.xml):

   <dependency>
       <groupId>com.rabbitmq</groupId>
       <artifactId>amqp-client</artifactId>
       <version>5.16.0</version> <!-- Use the latest version -->
   </dependency>

Producer Code (Sends the message):

import com.rabbitmq.client.Connection;
import com.rabbitmq.client.Channel;
import com.rabbitmq.client.ConnectionFactory;
public class RabbitMQProducer {
    private final static String QUEUE_NAME = "hello";
    public static void main(String[] argv) throws Exception {
        // 1. Create a connection factory
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost"); // RabbitMQ server address
        try (Connection connection = factory.newConnection();
             Channel channel = connection.createChannel()) {
            // 2. Declare a queue (idempotent - won't create if it already exists)
            channel.queueDeclare(QUEUE_NAME, false, false, false, null);
            // 3. Create the message
            String message = "Hello, this is a message from RabbitMQ!";
            // 4. Send the message to the queue
            channel.basicPublish("", QUEUE_NAME, null, message.getBytes());
            System.out.println(" [x] Sent '" + message + "'");
        }
    }
}

Consumer Code (Listens for and receives the message):

import com.rabbitmq.client.*;
import java.io.IOException;
public class RabbitMQConsumer {
    private final static String QUEUE_NAME = "hello";
    public static void main(String[] argv) throws Exception {
        ConnectionFactory factory = new ConnectionFactory();
        factory.setHost("localhost");
        try (Connection connection = factory.newConnection();
             Channel channel = connection.createChannel()) {
            channel.queueDeclare(QUEUE_NAME, false, false, false, null);
            System.out.println(" [*] Waiting for messages. To exit press CTRL+C");
            // 1. Create a DeliverCallback to handle incoming messages
            DeliverCallback deliverCallback = (consumerTag, delivery) -> {
                String message = new String(delivery.getBody(), "UTF-8");
                System.out.println(" [x] Received '" + message + "'");
            };
            // 2. Start consuming messages from the queue
            channel.basicConsume(QUEUE_NAME, true, deliverCallback, consumerTag -> { });
        }
    }
}

How to run it:

1. Run the RabbitMQConsumer. It will wait for messages.
2. Run the RabbitMQProducer. The consumer will immediately print the received message.

Sending Notifications: Sending an Email

A very common business requirement is to "send a message" via email. The standard library for this in Java is JavaMail.

Prerequisites:

1. Download the JavaMail API and the Jakarta Mail implementation (if using Java 11+).
   • Maven Dependency:

     <dependency>
         <groupId>com.sun.mail</groupId>
         <artifactId>jakarta.mail</artifactId>
         <version>2.0.1</version> <!-- Use the latest version -->
     </dependency>

2. You'll need access to an SMTP server (like Gmail, Outlook, or your company's server). For Gmail, you may need to allow "less secure apps" or create an App Password.

Code Example:

import jakarta.mail.*;
import jakarta.mail.internet.InternetAddress;
import jakarta.mail.internet.MimeMessage;
import java.util.Properties;
public class EmailSender {
    public static void sendEmail(String to, String subject, String text) {
        // 1. Set up mail server properties
        String host = "smtp.gmail.com"; // e.g., smtp.gmail.com
        Properties properties = System.getProperties();
        properties.put("mail.smtp.host", host);
        properties.put("mail.smtp.port", "587");
        properties.put("mail.smtp.auth", "true");
        properties.put("mail.smtp.starttls.enable", "true"); // For TLS
        // 2. Get the Session object
        // Replace with your email and password/app password
        final String username = "your-email@gmail.com";
        final String password = "your-app-password";
        Session session = Session.getInstance(properties, new Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        });
        try {
            // 3. Create a MimeMessage object
            MimeMessage message = new MimeMessage(session);
            message.setFrom(new InternetAddress(username));
            message.addRecipient(Message.RecipientType.TO, new InternetAddress(to));
            message.setSubject(subject);
            message.setText(text);
            // 4. Send the message
            Transport.send(message);
            System.out.println("Sent message successfully....");
        } catch (MessagingException mex) {
            mex.printStackTrace();
        }
    }
    public static void main(String[] args) {
        sendEmail("recipient@example.com", "Test Subject", "This is a test email sent from Java.");
    }
}

Summary: Which One Should You Use?