Datagram

Since this is an interface class, the internal structure of the datagram packets is not defined here. However, it is assumed that each implementation of this interface will provide the following fields / state variables (the actual implementation and the names of these fields may vary):

• buffer: the internal buffer in which data is stored
• offset: the read/write offset for the internal buffer
• length: the length of the data in datagram packet
• address: the destination or source address
• read/write pointer: a pointer that is added to the offset to point to the current data location during a read or write operation

Reading and Writing

The Datagram interface extends interfaces DataInput and DataOutput in order to provide a simple way to read and write binary data in and out of the datagram buffer instead of using getData and setData methods. Writing automatically increments length and reading will continue while the read/write pointer is less than length. Before any writing is done reset must be called. If setData() is to be used when reading or writing, any value for the offset parameter other than 0 is not supported.

For example to write to datagram:

datagram = connection.newDatagram(max);

// Reset prepares the datagram for writing new message.
datagram.reset();

// writeUTF automatically increases the datagram length.
datagram.writeUTF("hello world");

connection.send(datagram);

datagram = connection.newDatagram(max);

connection.receive(datagram);

message = datagram.readUTF();

It should be noted the length above is returned from getLength and can have different meanings at different times. When sending length is the number of bytes to send. Before receiving length is the maximum number of bytes to receive. After receiving length is the number of bytes that were received. So when reusing a datagram to receive after sending or receiving, length must be set back to the maximum using setLength.

datagram = connection.newDatagram(max);

while (notDone) {

// The last receive in the loop changed the length
// so put it back to the maximum length.
datagram.setLength(max);
connection.receive(datagram);

data = datagram.getData();
bytesReceived = datagram.getLength();

// process datagram ...
}

datagram = connection.newDatagram(max);

while (notDone) {

// The last read in the loop changed the read pointer
// so reset the pointer.
datagram.reset();
datagram.setLength(max);
connection.receive(datagram);

message = datagram.readUTF(message);

// process message ...
}

connection.receive(datagram);

message = datagram.readUTF(message);

len = datagram.getLength();

datagram.reset();

datagram.setLength(len);

copy = datagram.readUTF(message);