import java.io.*;
import java.net.*;
class TCPClient {
     public static void main (String argv[]) throws Exception
   {
      String sentence;
      String modifiedSentence;
      BufferedReader inFromUser = new BufferedReader( new InputStreamReader (System.in) );
      Socket clientSocket = new Socket ("hostname" , 6789);
      DataOutputStream outToServer = new DataOutputStream( clientSocket.getOutputStream() );
      BufferedReader inFromServer = new BufferedReader(new InputStreamReader(
                   clientSocket.getInputStream() ) );
      sentence = inFromUser.readLine();
      outputToServer.writeBytes (sentence + '\n');
      modifiedSentence = inFromServer.readLine();
      System.out.println("FROM SERVER: " + modifiedSentence);
      clientSocket.close();
    }
}

TCPServer.java

import java.io.*;
import java.net.*;
class TCPServer {
  public static void main (String argv[] ) throws Exception
        {
             String clientSentence;
             String capitalizedSentence;
             ServerSocket welcomeSocket = new ServerSocket (6789);
             while (true)   {
                  Socket connectionSocket = welcomeSocket.accept();
                  BufferedReader inFromClient =
                       new BufferedReader (new InputStreamReader(
                            connectionSocket.getInputStream() ) );
                  DataOutputSteam( connectionSocket.getOutputStream() );
                  clientSentence = inFromClient.readLine();
                  captitalizedSentence = clientSentence.toUpperCase() + '\n';
                  outToClient.writeBytes (capitalizedSentence);
                }
          }
}