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In this lesson we're going to work on the client side.

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Let's create our class by clicking on simple_tcp -> New -> Class.

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I'm going to name it simply "Client". Let's start with adding our main method, the same way we did it for

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the server.

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So I'm going to type in main, press Ctrl + Space, and then Enter, and then I'm going to add the try-catch

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block - I'm going to type in "try" and then press Ctrl + Space, and then Enter. And then we're going to initiate

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new Client() in the try block. And then in the catch block I'm going to say e.PrintStackTrace(). We

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can create our client method

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now. It's going to be public Client() throws Exception

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- and in this method is going to be our application.

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So this is going to be our client side of our program.

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And the first thing we need to do here is to create a socket.

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This time we only have one type of socket.

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On the server side, we had two types of sockets: we had the server socket and then the regular communication

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socket.

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So on the client side we're only going to have a regular socket.

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This socket is going to be - we're going to use the Socket() method again.

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So it's going to be Socket socket = new Socket(); and then we're going to forward two arguments to this

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method.

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It's going to be the IP address of the server and the listening port

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- so the port number where our server is expecting our connection. I'm going to use localhost, since my

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server is on this machine. And I'm going to use 2020, which we defined as our port number.

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Additionally, I'm going to press Ctrl + Shift + O, to import the java.net.socket packet, so

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that we don't have that error anymore. And I'm going to print out a message in the console, saying

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"Successful connection to the server". This printout right here is useful to us because if something fails

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in this line we're going to be thrown to the catch block.

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So, by reaching this line of code - by seeing this sentence in our console - we know that we have successfully

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passed this line of code.

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So, this really reassures us that everything went well and that we actually are connected to our server.

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Next thing we need to do here is create the input and output buffers.

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Now luckily for us, they're exactly the same as on the server side.

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So you can just copy them, but I'm going to type them out real quick. For the incoming data,

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we're going to need BufferedReader.

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We're going to name that in_socket = new BufferedReader (new InputStreamReader - and then socket.getInput

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- socket.getInputStream -

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and then for the outgoing socket, it's going to be PrintWriter -

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PrintWriter = out_socket (new PrintWriter...

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There it is.

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I'm going to add a comment here just to separate this section,

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and press Ctrl + Shift + O to import all the all the required packets. All right.

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These are the basic requirements for our client-server communication.

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And now what we need to worry about is the protocol.

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A protocol is pretty much just a set of rules by which communication between two or more sides occurs.

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So we need to define who is the first to send a message, what kind of message, what kind of responses

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are expected, etc etc.

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For our simple little program we're going to have a simple little protocol.

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So the client is going to connect to the server, the server is going to say "Welcome!", the client is then

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going to say "Thanks!", and then they're going to disconnect.

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We have already programmed the server to do all of these things.

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So it's going to be listening for connections,

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and then once the connection arrives, it's going to accept the connection, and then right away it's going

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to send out "Welcome!"

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It's going to then receive the message from the client.

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It's going to display it in its console, and then it's going to close the socket.

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So socket.close() is actually the line where we initiate disconnection or - interruption of the connection.

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All right.

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Let's go back to our client.

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So this is where the protocol actually takes place. First,

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we need to receive the message from the server. We're going to store that message in a string called

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"message" - very original.

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We're going to read that message from our socket, with "readLine()", and then we can display that message

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in our console.

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We're going to say "Server says:" and then add the message.

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Now we need to send "Thank you" back to the server.

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We're going to do that by using our out_socket object and println,

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and we're going to say "Thanks!"

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That's actually pretty much it.

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So according to our protocol, we need to connect to the server, receive the "Welcome!" message, send a

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"Thanks!" message, and then disconnect.

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So we're going to disconnect by closing the socket: socket .close(), and we're going to

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print out a message in our console, saying "Socket closed". Now I'm going to save this and try and run it

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and see if it works.

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First, we're going to run the server, so you run the server by pressing - or choosing - the server class, and

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then pressing "Play" or "Run" in Eclipse.

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And then I'm going to choose the client class, and also press Run.

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So the program is going to just go through all of the code by itself, since we've already typed in all the

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messages that need to be sent.

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But let's just quickly go over what happened.

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In order to switch between console messages for server and client, you need to press this little icon

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of a display here.

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So right now we're looking at the server console, and we can see all of our messages.

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So ports 2020 is open.

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We got that right here.

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So we get that message

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once we successfully reserve a new port in our operating system, and then when a client connects, we get

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this message.

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This is the IP address of our client - 127.0.0.1 - that's the loopback address,

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and then we get "Client says: Thanks!"

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So we have read the client's message.

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But before that, we sent "Welcome!" to the client.

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So this actually happened before this.

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All right.

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As soon as these messages were exchanged, we've closed the sockets on both sides.

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So that's when we got "Socket is closed."

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message, both on the server and on the client.