Following up on my previous post, we also had to demonstrate a sample Java TCP Server and TCP Client. They are pretty small and give you a good idea about how a TDP Server opens up a port, and then the TCP Client sends or receives data from that port.

This is a good page on the differences between TCP and UDP.

To compile these, install Java JDK to your system. Then compile the program with “javac TCPClient.java” - this will create a TCPClient.class. Execute the file with “java TCPClient” - leave off the .class, or you will get the error: “Exception in thread “main” java.lang.NoClassDefFoundError”.

Here is the sample code:

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()));
            DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
            clientSentence = inFromClient.readLine();
            System.out.println("Received: " + clientSentence);
            capitalizedSentence = clientSentence.toUpperCase() + '\n';
            outToClient.writeBytes(capitalizedSentence);
         }
      }
}

and the client:

TCPClient.java

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("localhost", 6789);
  DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
  BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
  sentence = inFromUser.readLine();
  outToServer.writeBytes(sentence + '\n');
  modifiedSentence = inFromServer.readLine();
  System.out.println("FROM SERVER: " + modifiedSentence);
  clientSocket.close();
 }
}

If you have any questions, please leave a comment!

For a class I am taking, we are testing out a simple UDP Server and UDP Client to demonstrate what each one does and how sockets work. They are pretty small and give you a good idea about how a UDP Server opens up a port, and then the UDP Client sends or receives data from that port.

To compile these, install Java JDK to your system. Then compile the program with “javac UDPClient.java” - this will create a UDPClient.class. Execute the file with “java UDPClass” - leave off the .class, or you will get the error: “Exception in thread “main” java.lang.NoClassDefFoundError”.

Here is the sample code:

UDPServer.java:

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

class UDPServer
{
   public static void main(String args[]) throws Exception
      {
         DatagramSocket serverSocket = new DatagramSocket(9876);
            byte[] receiveData = new byte[1024];
            byte[] sendData = new byte[1024];
            while(true)
               {
                  DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
                  serverSocket.receive(receivePacket);
                  String sentence = new String( receivePacket.getData());
                  System.out.println("RECEIVED: " + sentence);
                  InetAddress IPAddress = receivePacket.getAddress();
                  int port = receivePacket.getPort();
                  String capitalizedSentence = sentence.toUpperCase();
                  sendData = capitalizedSentence.getBytes();
                  DatagramPacket sendPacket =
                  new DatagramPacket(sendData, sendData.length, IPAddress, port);
                  serverSocket.send(sendPacket);
               }
      }
}

UDPClient.java:

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

class UDPClient
{
   public static void main(String args[]) throws Exception
   {
      BufferedReader inFromUser =
         new BufferedReader(new InputStreamReader(System.in));
      DatagramSocket clientSocket = new DatagramSocket();
      InetAddress IPAddress = InetAddress.getByName("localhost");
      byte[] sendData = new byte[1024];
      byte[] receiveData = new byte[1024];
      String sentence = inFromUser.readLine();
      sendData = sentence.getBytes();
      DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
      clientSocket.send(sendPacket);
      DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
      clientSocket.receive(receivePacket);
      String modifiedSentence = new String(receivePacket.getData());
      System.out.println("FROM SERVER:" + modifiedSentence);
      clientSocket.close();
   }
}