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In this section, we talk about UDP socket programming in Java.

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First, let's remind ourselves of the difference between TCP and UDP.

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They are both transport-layer protocols, and the main difference between them is that TCP requires

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a connection to be established before communicating, but UDP doesn't.

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From the perspective of coding network applications, in both cases we use sockets because we need

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a door through which we're going to send data.

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So both in TCP and UDP, we use a socket, and we have port numbers.

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But the difference is that in UDP there is no clear establishment of a connection.

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We just send every packet separately and hope for the best.

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Seriously - that's how UDP works :)

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There is no flow control,

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congestion control, no sequence, no acknowledgement - none of that.

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We just fill out these four fields, and fill the payload with some data, send it out and hope for the

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best.

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All right. Let's create a new project for this.

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I'm going to do a right click in the Package Explorer area.

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Go to New -> Java Project.

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Let's call it Simple_UDP.

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Now we're going to need a new packet,

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and then I'm going to create two classes.

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We're going to have a Receiver

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and a Sender.

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We're going to create pretty much the same program we created in our first exercise, where the sender

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will send a message to the receiver, and then the receiver will send something back, and that's going

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to be it.

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We're going to start from the Receiver.

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The first thing we're going to need here is going to be - of course - the inevitable main method. And we're

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going to use try-catch here too. In the catch block,

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I'm going to use e.printStackTrace(), and then in the try block, I'm going to say new Receiver().

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Let's go and create that method.

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It's going to be public Receiver() and we need an exception here -

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we'll start from creating a DatagramSocket.

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Ss you can probably guess,

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This is similar to our server socket, because we are defining our port number here.

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That's because even though we don't technically have a client or a server, we still need to reserve a

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port number in our operating system, and also an application - or rather a network application - has to have

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a port number. Regardless,

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there is going to be a lesson where I'm going to go through the code again,

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so don't worry - I will clear up the details later.

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I'm going to press Ctrl + Shift + O to import the DatagramSocket package.

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And now we can move on.

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The first thing I'm gonna do is - I'm going to print out

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"Receiver is running", and then we're going to need a Scanner here just like we had in our previous examples.

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I'm going to name it "keyboard",

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and that's going to come from System input.

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And we're also going to need Ctrl + Shift + O

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here. Now comes the messy part.

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Since there are no streams in UDP, we have to pretty much build our own fields in the packet, and also

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the payload.

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So the payload is sent as bytes, and the other fields -

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- let's just go through them - source port: your computer knows which source sport it's using,

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so we don't need to define that, but we do need to define destination port.

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We do need to define the length of the data, and then "Header and Data Checksum" also is done by your

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operating system.

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So there are some tricky things here so you're going to have to just bear with me and I promise to explain

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all of this code in even more detail in a separate lesson.

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We're going to create a buffer here, and this buffer is going to be an array of bytes.

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I'm going to say >>byte[]<< and we're gonna call that "buffer", and we're going to say "new bytes", and the size

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of this array is 1500.

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This number is not accidental,

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it's called the MTU - Maximum Transmission Unit, and 1500 bytes is the limit of how much

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data you can transfer in a single packet.

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Now we need to create a packet.

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Think of it as - creating a frame where you're going to place your data. So it's DatagramPacket -

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We're going to call it - packet = new DatagramPacket, and then we need the buffer in which we're going

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to place the data, and then we need buffer length... Since we have created an array of 1500 bytes,

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we could just say "1500" instead of "buffer.length", but let's do it properly.

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I'm going to press Ctrl + Shift + O to

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import the DatagramPacket package, and then we're going to move on to socket.receive(). At this point,

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we are receiving the message from the Sender, and we want to place that message into our newly created

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packets.

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I'm going to add a comment here -

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and now we need to extract a string from that message.

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So our message is in this object, and the message is actually in this buffer part.

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We want to create a string.

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We're going to call that string "message" and we're going to say new String - take the data from buffer,

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and then trim that data. We're using trim() here because there's a huge possibility that the sender typed

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in something much shorter than 1500 bytes. If you remember from one of our previous examples,

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"Welcome!" was only 10 bytes.

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So we're using this method to get rid of any unnecessary data that comes after our sender's message.

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Now that we've got the string, there's nothing stopping us from printing that string out.

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We're going to say "Received: " -

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and then display the message. At this point we're pretty much done with the receiving part.

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Now we need to focus on the sending part. In order to know where to send our response,

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we need to get the IP address of the sender, and the port number that the sender used. Here I'm talking

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about the source number that the sender used, only this time, we're going to use their source number as

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our destination number.

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This data is stored in the packet itself.

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I'm going to say InetAddress senders_address - and then get that from the packet.

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And then senders_port is an integer -

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.getPort();

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Ctrl + Shift + O to import the packages, and we can focus on the sending part

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now. We want our user to enter their message,

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and then we're going to store their message into our message string.

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We're going to say keyboard -

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now we need to switch that message - or that string - into bytes.

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We're going to fill our buffer with that message by saying "message" - and then .getBytes() out of my message.

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Now we need to fill out our packets.

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We're going to create a new packet, and what we need is the buffer.

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Our message is actually in the buffer.

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We need to say what's the length of our data.

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We need to say what's -

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OK.

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- we need to say what's the IP address where we're sending the message, and also what's the port. All there

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is left to do is to send out that packet.

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We're going to use socket.send -

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and then we're sending our packet.

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I'm going to add a printout just to track where we are.

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We're going to say "Sent: " and then display the message that we sent.

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That's it for the receiver. Of course to be able to test this program,

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we need to create the sender.

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So we'll do that in the next lesson.

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See you there.