Today we are going to create a server which is going to perform factorial of a number for its client. For this task, we are going to take the help of the JAVA programming language and its different libraries. We shall also understand how a socket server runs and connect with different clients. 



We created a server socket object on the server side which is running in port number 6666 of localhost (127.0.0.1). Then we created a socket object on the client side which will try to connect to port 6666. After the arrival of a successful request from the client side, the server will accept the request creating another socket object on their side.
Finally, the connection is established. Now we will send a number from the client side to the server, and a method named, "factorial" will be called server s  ide. In the end, the returned value will be printed on the client side.


Let me create our server first...

//server

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

public class b {
    public static void main(String args[]) throws IOException
    {
        System.out.println("...welcome...");
        ServerSocket ss = new ServerSocket(6666);
        System.out.println("...factorial server running...");
        int a = 0;
        while(true)
        {
            a=a+1;
            System.out.println("...client searching...");
            Socket sc = ss.accept();
            System.out.println("...client found...");
            DataInputStream inp = new DataInputStream(sc.getInputStream());
            String msg = inp.readUTF();
            int num = Integer.parseInt(msg);
            System.out.println("request-"+a+": fac(" + num+")");
            DataOutputStream out = new DataOutputStream(sc.getOutputStream());
            out.writeUTF("fac("+num+") is "+factorial(num));
        }
    }
    public static long factorial(int n) {
        long fac = 1;
        if (n==0 || n==1) {
            return fac;
        }
        else{
            while(n>1){
                fac = fac*n;
                n = n-1;
            }
            return fac;
        }        
    }
}

Now create our client...

//client

import java.io.*;
import java.net.*;
public class a {
    public static void main(String args[]) throws IOException
    {
        Socket s = new Socket("localhost", 6666);
        System.out.println("...connection established...");
        boolean valid = false;
        int num = 0;
        while(valid != true){
            System.out.print("enter any number: ");
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            try {
                num = Integer.parseInt(br.readLine());
                valid=true;
            } catch (Exception e) {
                valid=false;
                System.out.println("...please enter only numbers...");
            }
        }
       
        DataOutputStream out = new DataOutputStream(s.getOutputStream());
        out.writeUTF(Integer.toString(num));
        DataInputStream inp = new DataInputStream(s.getInputStream());
        String msg = inp.readUTF();
        System.out.println("server: "+ msg);

        s.close();
    }
}


If you want to understand the line-by-line analysis of these two programs, just check out this video.