I haven't found an exact answer to the problem that I'm having, so I'm going to ask this anyway, and if I did, in fact, repost a question that's already been asked, I apologize.

I'm doing another lab assignment for my Java class, and for this week's lab, I have to make a UDP server to send out a Message object across the network, then I have to make a UDP client to read in that Message. The Message itself is just an Object that has a String message and a String username; fairly arbitrary. Message is Serializable.

Now, what I'm having an issue with is how to actually go about sending that Message Object across the network within a DatagramPacket? The actual constructor has me put in a byte array, the size of the array, the InetAddress, and the port number. My final question is: how to I find out the size of the array, and how to I turn my Message into bytes to be sent out?

You need to serialize your message class into a byte array. This will be the byte array you send (and it will be easy to get the size at that point).

On the client, you will want to deserialize the byte array back into a Message object.

Java provides a set of classes to handle serialization/deserialization, and the object you want to serialize must implement the "Serializable" interface.

Something like this would work:

// Serialize to a byte array
ByteArrayOutputStream bStream = new ByteArrayOutputStream();
ObjectOutput oo = new ObjectOutputStream(bStream); 
oo.writeObject(messageClass);
oo.close();

byte[] serializedMessage = bStream.toByteArray();

And on the receiving end:

ObjectInputStream iStream = new ObjectInputStream(new ByteArrayInputStream(recBytes));
Message messageClass = (Message) iStream.readObject();
iStream.close();

Note, you should create a common interface that is shared between the client and the server, this will allow for easy serialization/deserialization of the payload.