gistfile1.java

package server;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;

/**
*
* @author Rashid
*/
public class Server {
    public static void main(String[] args ) throws Exception{

        String clientSentence; 
        String capitalizedSentence; 

        // Creating Server Port 6789
        ServerSocket welcomeSocket = new ServerSocket(6789); 
        System.out.println("Server Started and listening to the port 6789");
        //always use try-catch to catch issues and trouble rather than crashing your apps.
        //also use finally to close the connection after closing the server, or it will keep running
        try{
            while(true) { 

                /*This part looks TOO COMPLEX, i am going to improve it       
                // The server accepts the waiting client
                Socket connectionSocket = welcomeSocket.accept(); 
                // IN           
                BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); // OUT
                DataOutputStream  outToClient = new DataOutputStream(connectionSocket.getOutputStream()); 
                */

                Socket socket = welcomeSocket.accept();
                //again try-catch-finally is essential here too
                try{
                    // READ LINE
                    //clientSentence = inFromClient.readLine();
                    //INPUT and OUTPUT Streams..
                    BufferedReader in = new BufferedReader(
                        new InputStreamReader(socket.getInputStream()));

                    PrintWriter out =
                    new PrintWriter(socket.getOutputStream(), true);

                    clientSentence = in.readLine();
                    if (input == null || input.equals(".")) {
                        break;
                    }
                    // UPPER CASE 
                    capitalizedSentence = clientSentence.toUpperCase() + '\n'; 
                    out.println(capitalizedSentence);
                    //outToClient.writeBytes(capitalizedSentence); 
                }
                catch (IOException e) {
                    System.out.println("An error has occured : " + e);
                }
                finally{
                    socket.close();
                }
            }
        }
        finally{
            welcomeSocket.close();
        }
    } 
} 

gistfile2.java

package tcp;
import java.io.*; 
import java.net.*; 
 
public class TCP {
    public static void main(String[] args) throws Exception  {
		String Sentence;
		String modifiedSentence;

		// File Reading
		//-----------------

		// -----------------

		// Open Connection to the Port 6789
		// First Parameter is Server-Address and Second: is the  Port
	    Socket clientSocket = new Socket("localhost",6789);
		BufferedReader inFromUser =  new BufferedReader(new InputStreamReader(System.in)); 
	 
		PrintWriter toServer = new PrintWriter(clientSocket.getOutputStream(), true);

	        
	    toServer.println(inFromUser);
	    BufferedReader fromServer = new BufferedReader(
	        	new InputStreamReader(clientSocket.getInputStream()));
	 	while (true) {
	 		if (fromServer.readLine() != null) {
	 			System.out.println("Server response : " + fromServer.readLine());
	 			break;
	 		}
	 	}
    }
}