• Socket class

public Socket(String host, int port)
        throws UnknownHostException, IOException

• getInputStream
  Returns the input stream of Socket, and the user accepts the data from remote.
  
• getOutputStream
  Returns the output stream of Socket for sending data.
  
• accept

public Socket accept() throws IOException

• Constructor

public ServerSocket(int port) throws IOException

Example:
Server side:

public class Server {
    public static void main(String[] args) throws IOException {

        //1. Create ServerSocket
        ServerSocket serverSocket = new ServerSocket(1234);

        //2. monitoring
        Socket socket = serverSocket.accept();
        System.out.println("server start listen");

        //3. input stream (send dy client)
        InputStream is = socket.getInputStream();
        InputStreamReader reader = new InputStreamReader(is);
        BufferedReader br = new BufferedReader(reader);
        String content = null;
        // construct the sting buffers
        StringBuffer sb = new StringBuffer();
        while ((content = br.readLine()) != null) {
            sb.append(content);
        }
        
        // 4. You can write your logic here to do
        //what you want those message do.
        System.out.println("server receiver: " + sb.toString());

        // Output stream
        OutputStream os = socket.getOutputStream();
        // Send data
        os.write("Server Hello".getBytes());
        System.out.println("send message");
        os.flush();

        // 5. pause the monitor socket, then close related buffers
        socket.shutdownInput();
        // Close output stream
        os.close();
        // Close buffer reader
        br.close();
        // Close stream reader
        reader.close();
        // Close input stream
        is.close();
        // Close monitor socket
        socket.close();
        // Close server socket
        serverSocket.close();
    }
}

Client side:

public class Client{
        //1. Create Client
        Socket socket = new Socket("your ip", 1234);
        //2. output stream
        OutputStream os = socket.getOutputStream();
        //3. Send data
        os.write("Hello world".getBytes());
        System.out.println("send message");
        os.flush();
        // 4. pause socket, then close related buffers
        socket.shutdownOutput();
        os.close();
        socket.close();
}

Ref.
https://hit-alibaba.github.io/interview/basic/network/Socket-Programming-Basic.html
https://programming.vip/docs/android-network-programming-socket.html