Sockets are a fundamental technology for network communication in Java. They provide a way for programs to communicate over a network, enabling the exchange of data between a client and a server. In this section, we will cover the basics of sockets, how to create and use them in Java, and provide practical examples and exercises to solidify your understanding.

1. Socket: An endpoint for communication between two machines.
2. ServerSocket: A class used by server applications to obtain a port and listen for client requests.
3. Client-Server Model: A model where the server provides resources or services, and the client accesses them.

To create a socket in Java, you can use the Socket class for the client side and the ServerSocket class for the server side.

• Socket client = new Socket(serverName, port);: Creates a socket to connect to the specified server and port.
• OutputStream outToServer = client.getOutputStream();: Gets the output stream of the socket to send data to the server.
• DataOutputStream out = new DataOutputStream(outToServer);: Wraps the output stream to send data in a specific format.
• out.writeUTF("Hello from " + client.getLocalSocketAddress());: Sends a UTF-8 encoded string to the server.
• InputStream inFromServer = client.getInputStream();: Gets the input stream of the socket to receive data from the server.
• DataInputStream in = new DataInputStream(inFromServer);: Wraps the input stream to read data in a specific format.
• System.out.println("Server says " + in.readUTF());: Reads and prints the response from the server.

• ServerSocket serverSocket = new ServerSocket(port);: Creates a server socket that listens on the specified port.
• Socket server = serverSocket.accept();: Waits for a client to connect and returns a socket for communication.
• DataInputStream in = new DataInputStream(server.getInputStream());: Gets the input stream of the socket to receive data from the client.
• System.out.println(in.readUTF());: Reads and prints the message from the client.
• DataOutputStream out = new DataOutputStream(server.getOutputStream());: Gets the output stream of the socket to send data to the client.
• out.writeUTF("Thank you for connecting to " + server.getLocalSocketAddress() + "\nGoodbye!");: Sends a UTF-8 encoded string to the client.

Create a simple client-server application where the client sends a message to the server, and the server responds with a confirmation message.

1. Client.java

   import java.io.*;
   import java.net.*;
   
   public class Client {
       public static void main(String[] args) {
           String serverName = "localhost";
           int port = 8080;
   
           try {
               Socket client = new Socket(serverName, port);
               OutputStream outToServer = client.getOutputStream();
               DataOutputStream out = new DataOutputStream(outToServer);
   
               out.writeUTF("Hello Server!");
               InputStream inFromServer = client.getInputStream();
               DataInputStream in = new DataInputStream(inFromServer);
   
               System.out.println("Server says: " + in.readUTF());
               client.close();
           } catch (IOException e) {
               e.printStackTrace();
           }
       }
   }
   

2. Server.java

   import java.io.*;
   import java.net.*;
   
   public class Server {
       public static void main(String[] args) {
           int port = 8080;
   
           try {
               ServerSocket serverSocket = new ServerSocket(port);
               System.out.println("Server started and listening on port " + port);
   
               while (true) {
                   Socket server = serverSocket.accept();
                   DataInputStream in = new DataInputStream(server.getInputStream());
                   System.out.println("Client says: " + in.readUTF());
                   DataOutputStream out = new DataOutputStream(server.getOutputStream());
                   out.writeUTF("Message received!");
                   server.close();
               }
           } catch (IOException e) {
               e.printStackTrace();
           }
       }
   }
   

• Not closing the socket: Always close the socket after communication to free up resources.
• Incorrect port number: Ensure the client and server use the same port number.
• Handling exceptions: Properly handle exceptions to avoid crashes and provide meaningful error messages.

In this section, we covered the basics of sockets in Java, including how to create client and server sockets, and how to send and receive data. We also provided a practical exercise to reinforce the concepts. Understanding sockets is crucial for network programming and will serve as a foundation for more advanced topics in networking.