Socket Programming in JAVA

Whats a socket?

Every computer on a network has an IP address. This address is   like your house address, something that identifies your   computer uniquely, allowing others  to communicate with your  computer. I wont go much into IP addresses, but let me  just tell you that  an IP 

address looks something like this - 64.104.137.158  a set of  numbers separated with dots.

However, some computers with rich owners will also choose to  have a domain name in addition to this sick looking number, so  that people can easily identify them. 

A domain name looks far more sane - like www.yahoo.com.There  are some special computer  on the Internet, whose sole purpose  in life is to translate the domain name to IP address and vice  versa. 

 An IP address = uniquely identifies a computer on the network.

 A port number = uniquely identifies a program running on a computer.

 An IP address + A port number = SOCKET

a socket is a combination of an IP address and a port. A socket address lets other computers on the network locate a  certain program running on a certain computer. You may represent a socket address  like 64.104.137.58:80, where 64.104.137.58 is the IP address and 80 is the port number.

Setps For Program

1: One Java program will try to connect to another Java program. 

   Lets call the  first program   

                   as Client, 

   and the second 

                   as   Server.

 
2: Once connected, the client program is going to accept whatever you type,  and send it 

    dutifully to the server program.

 
3: The server is going to send back the same text to the client, just to show  that it is least 

    interested in doing such an uninteresting thing.

 
 Create 2 fresh Java programs and call them Server.java and Client.java. 

Server.java

import java.net.*;

import java.io.*; 

public class Server

{ 

public static void main(String[] ar) 

{ 

 int port = 6666; // just a random port.  

 try 

  { 

   ServerSocket ss = new ServerSocket(port); // create a server socket and bind 

   System.out.println("Waiting for a client..."); 

   Socket socket = ss.accept(); // make the server listen for a connection     

   System.out.println("Client Recived."); 

   System.out.println();  /* Get the input and output streams of the socket, so that you can receive and send data to the client.*/ 

   InputStream sin = socket.getInputStream();

   OutputStream sout = socket.getOutputStream();  /*Just converting them to different streams, so that string handling becomes   easier.*/  

   DataInputStream in = new DataInputStream(sin); 

   DataOutputStream out = new DataOutputStream(sout);

   String line = null; 

   while(true)

   { 

    line = in.readUTF(); // wait for the client to send a line of text.

    System.out.println("Data from Client: " + line);

    System.out.println("I'm sending it back...");

    out.writeUTF(line); // send the same line back to the client. 

    out.flush(); //flush the stream .  

    System.out.println("Waiting for the next line..."); 

    System.out.println(); 

   }

  } 

  catch(Exception x)

  { 

  x.printStackTrace(); 

   }

 }

} 

Client.java

import java.net.*;

import java.io.*;

public class Client 

{

public static void main(String[] ar)

 {

 int serverPort = 6666; // make sure you give the port number on which the server is listening.

 String address = "127.0.0.1"; // this is the IP address of the server program's computer. 

 try 

{

 InetAddress ipAddress = InetAddress.getByName(address); // create an object that represents the above IP address.

 System.out.println("a socket with IP address " + address + " and port " + serverPort + "?");

 Socket socket = new Socket(ipAddress, serverPort); // create a socket with the server's IP address and server's port.

 System.out.println("----Connect-----");

 /* Get the input and output streams of the socket, so that you can receive and send data to the client*/

 InputStream sin = socket.getInputStream();

 OutputStream sout = socket.getOutputStream();

 // Just converting them to different streams, so that string handling becomes easier.

 DataInputStream in = new DataInputStream(sin);

 DataOutputStream out = new DataOutputStream(sout);

 // Create a stream to read from the keyboard.

 BufferedReader keyboard = new BufferedReader(new InputStreamReader(System.in));

 String line = null;

 System.out.println("Type in something and press enter. ");

 System.out.println();

 while(true)

 {

 line = keyboard.readLine(); // wait for the user to type in something and press enter.

 System.out.println("Sending this line to the server...");

 out.writeUTF(line); // send the above line to the server.

 out.flush(); // flush the stream to ensure that the data reaches the other end.

 line = in.readUTF(); // wait for the server to send a line of text.

 System.out.println("The server was very polite. It sent me this : " + line);

 System.out.println("Looks like the server is pleased with us. Go ahead and enter more lines.");

 System.out.println();

 }

 } catch(Exception x) {

 x.printStackTrace();

 }

 }

 }

 Compile it with 

  javac Server.java Client.java

Open two command windows (DOS prompts). In one of those, enter

                java Server

and in the other,

                java Client