Simple Calculator via UDP in Java

Networking is a two-way communication between a host and a server, rather an exchange of information. Any server is setup to respond to specific queries as requested by a client. There are many different kinds of server based on the utility they provide to the users. Some examples include File Server, Application Server, Domain Server etc.

Let’s take an example to demonstrate how a request by any client is handled in the client-server model of communication. Assume that there is a company X which has hundreds of employees each working on their own system and most of them use printer to print some report or invoice etc. Installing printer on every system is not a very economical solution to the problem of providing printing facility to the users. An alternative to this is to setup a Print Server and install a printer on that system. When any employee wants to print something, he or she can send a request to the server asking to reserve the printer for him/her. The server on receiving the request has to make a decision based on many factors like availability of printer, rate of incoming requests etc, and finally respond to the request telling the client about the status of printer. One fundamental aspect of any server is that it should be specific, not generic. It must not generate similar response to every request rather respond based on the nature of request, the client to which it is responding etc.

This article is the implementation of simple calculator-server via UDP wherein the client will send the mathematical equation to the server and the server will respond with the answer to equation.



Client Side Programming

For sending or receiving over the Internet, there is a need to know the socket address of the listening entity, that is IP address and port number of the listener. Generally, the client knows, or server relays the socket address to which it is listening. So on the client side there is a very little change in the code part.
Instead of invoking only a send() call, the client also invokes receive() call. Rest all steps remain the same.
Steps involved on client side:

  1. Creation of DatagramSocket:- First, a datagramSocket object is created to carry the packet to the destination and to receive it whenever the server sends any data.
  2. Creation of DatagramPacket: In this step, the packet for sending/receiving data via a datagramSocket is created.
    Note : There is a difference in constructors for creating a datagram packet for sending and receiving the data
  3. Invoke a send() call on socket object : This would send our request carrying the equation to the server for processing.
  4. Invoke a receive() call on socket object : This is used to receive the data sent by the server after processing our request. This would freeze our program until the server responds or throws an error if it takes too much time.
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// Java Program to illustrate Client Side implementation
// of Simple Calculator using UDP
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.Scanner;
  
public class Calc_Client_UDP
{
    public static void main(String args[]) throws IOException
    {
        Scanner sc = new Scanner(System.in);
  
        // Step 1:Create the socket object for carrying
        // the data
        DatagramSocket ds = new DatagramSocket();
  
        InetAddress ip = InetAddress.getLocalHost();
        byte buf[] = null;
  
        // loop while user not enters "bye"
        while (true)
        {
            System.out.print("Enter the equation in the format:");
            System.out.println("'operand1 operator operand2'");
            String inp = sc.nextLine();
            buf = new byte[65535];
  
            // convert the String input into the byte array.
            buf = inp.getBytes();
  
            // Step 2:Create the datagramPacket for sending the data.
            DatagramPacket DpSend =
                      new DatagramPacket(buf, buf.length, ip, 1234);
  
            // invoke the send call to actually send the data.
            ds.send(DpSend);
  
            // break the loop if user enters "bye"
            if (inp.equals("bye"))
                break;
  
            buf = new byte[65535];
            DatagramPacket DpReceive =
                                 new DatagramPacket(buf, buf.length);
            ds.receive(DpReceive);
  
            System.out.println("Answer = " +
                                new String(buf,0,buf.length));
        }
    }
}

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Output:

Enter the equation in the form: 'operand operator operand'
5 * 6
Answer=30
Enter the equation in the form: 'operand operator operand'
5 + 6
Answer=11
Enter the equation in the form: 'operand operator operand'
9 / 3
Answer=3

Server Side Programming


As the socket address is required to communicate over the internet, the server must know the address through which the client is sending the request. Lets look step by step how server handles the problem of port number and respond to the queries of the client.
Steps involved on Server Side:

  1. Establish a socket connection.
  2. Process the equations coming from client: In server side also we open both the inputStream and outputStream. After receiving the equation, we process it save the result to be sent back to the client.
  3. Creating a packet for sending the result : This step creates a problem for the server as it does’nt know the port number of the client. To get the port, we use the following method of DatagramPacket class.
    public int getPort()
    Syntax:

    public int getPort()
    Returns the port number to which the specified datagram packet is being sent to or
    from which the packet is received.
    

  4. Close the connection.
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// Java Program to illustrate Server Side implementation
// of Simple Calculator using UDP
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.StringTokenizer;
  
public class Calc_Server_UDP
{
    public static void main(String[] args) throws IOException
    {
        // Create a socket to listen at port 1234
        DatagramSocket ds = new DatagramSocket(1234);
        byte[] buf = null;
        DatagramPacket DpReceive = null;
        DatagramPacket DpSend = null;
        while (true)
        {
            buf = new byte[65535];
  
            // create a DatgramPacket to receive the data.
            DpReceive = new DatagramPacket(buf, buf.length);
  
            // receive the data in byte buffer.
            ds.receive(DpReceive);
  
            String inp = new String(buf, 0, buf.length);
  
            //To remove extra spaces.
            inp=inp.trim();
            System.out.println("Equation Received:- " + inp);
  
            // Exit the server if the client sends "bye"
            if (inp.equals("bye"))
            {
                System.out.println("Client sent bye.....EXITING");
                break;
            }
  
            int result;
  
            // Use StringTokenizer to break the
            // equation into operand and operation
            StringTokenizer st = new StringTokenizer(inp);
  
            int oprnd1 = Integer.parseInt(st.nextToken());
            String operation = st.nextToken();
            int oprnd2 = Integer.parseInt(st.nextToken());
  
            // perform the required operation.
            if (operation.equals(" + "))
                result = oprnd1 + oprnd2;
  
            else if (operation.equals(" - "))
                result = oprnd1 - oprnd2;
  
            else if (operation.equals(" * "))
                result = oprnd1 * oprnd2;
  
            else
                result = oprnd1 / oprnd2;
  
            System.out.println("Sending the result...");
            String res = Integer.toString(result);
  
            // Clear the buffer after every message.
            buf = res.getBytes();
  
            // get the port of client.
            int port = DpReceive.getPort();
  
            DpSend = new DatagramPacket(buf, buf.length,
                          InetAddress.getLocalHost(), port);
            ds.send(DpSend);
        }
    }
}

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Output:

Equation received:-5 * 6
Sending the result...
Equation received:-5 + 6
Sending the result...
Equation received:-9 / 3
Sending the result...

Note: In order to test the above programs on the system, please make sure that you run the server program first and then the client one. Make sure you are in the client console and from there enter the equation in the format-“operand1 operator operand2” and press Enter. Answer to the requested equation will be shown in the client console only. Finally to terminate the communication, type “bye” (without quotes) and hit enter.

Reference: Java Official Documentation

Related Article:
Simple Calculator using TCP in Java

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