Java provides a comprehensive set of networking APIs (Application Programming Interfaces) that enable developers to write programs that can communicate over a network. Some common tasks that can be performed using Java’s networking APIs include:

1. Connecting to a server and sending and receiving data
2. Setting up a server and listening for incoming client connections
3. Reading and writing files over a network
4. Sending email
5. Performing DNS (Domain Name System) lookups

Here are some of the main classes and interfaces in the Java networking APIs:

• java.net.URL: Represents a uniform resource locator (URL) and can be used to create a connection to a remote resource.
• java.net.HttpURLConnection: Represents a connection to a URL for reading or writing data.
• java.net.Socket: Represents a connection between a client and a server.
• java.net.ServerSocket: Represents a server-side socket that listens for incoming client connections.
• java.net.DatagramSocket: Represents a socket for sending and receiving datagram packets.
• java.net.InetAddress: Represents an IP (Internet Protocol) address.

Here is an example of how to use the java.net.Socket class to create a client that connects to a server and sends a message:

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

public class Client {
    public static void main(String[] args) {
        String host = "localhost";
        int port = 8080;
        String message = "Hello, World!";

        try (Socket socket = new Socket(host, port)) {
            OutputStream output = socket.getOutputStream();
            PrintWriter writer = new PrintWriter(output, true);
            writer.println(message);

            InputStream input = socket.getInputStream();
            BufferedReader reader = new BufferedReader(new InputStreamReader(input));
            String response = reader.readLine();
            System.out.println(response);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

And here is an example of how to use the java.net.ServerSocket class to create a server that listens for incoming client connections and echoes back any messages it receives:

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

public class Server {
    public static void main(String[] args) {
        int port = 8080;

        try (ServerSocket serverSocket = new ServerSocket(port)) {
            while (true) {
                Socket clientSocket = serverSocket.accept();
                OutputStream output = clientSocket.getOutputStream();
                PrintWriter writer = new PrintWriter(output, true);

                InputStream input = clientSocket.getInputStream();
                BufferedReader reader = new BufferedReader(new InputStreamReader(input));
                String message = reader.readLine();
                writer.println("Echo: " + message);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

For more information on the Java networking APIs, you can refer to the official Java documentation at https://docs.oracle.com/en/java/javase/14/docs/api/java.