TCPServer.java

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

class TCPServer { 

	public static void main(String argv[]) throws Exception 
	{ 
		String clientSentence; 
		String capitalizedSentence; 

		/* Create a "listening socket" on the specified port */
		ServerSocket welcomeSocket = new ServerSocket(6789); 

		while(true) { 
			/* 
			 * Call to accept function (blocking call).
			 * When a new connection arrives a new "established socket" is created
			 */
			Socket connectionSocket = welcomeSocket.accept(); 

			/* Initialize input stream from the socket */
			BufferedReader inFromClient = 
				new BufferedReader(new
						InputStreamReader(connectionSocket.getInputStream())); 

			/* Initialize output stream towards the socket */
			DataOutputStream  outToClient = 
				new DataOutputStream(connectionSocket.getOutputStream()); 

			/* Read a line (ending with '\n') from the client */
			clientSentence = inFromClient.readLine(); 

			/* Build the response */
			capitalizedSentence = clientSentence.toUpperCase() + '\n'; 

			/* Send the response to the client */
			outToClient.writeBytes(capitalizedSentence);

			connectionSocket.close();

		}
	}
}