Programming Tutorials

Datagrams are based on a connection-less paradigm, which means that a conversation is not established between two systems. Instead, datagrams are blindly transmitted across a network connection. To transmit a datagram from one system to another, an application creates a datagram and sends it to the intended target. This is a rather simple process in J2ME. However, there is a caveat to using datagrams in that they offer no guarantee or acknowledgement as to whether the datagram actually reached the intended recipient. As a result, a datagram implementation must either not care if the recipient actually receives the data 100 percent of the time, or an acknowledgement/handshake must be implemented manually by the application.

Using Datagrams
The following example shows how to create a datagram and send it to a specific IP address.

try	{
  DatagramConnection dgc = (DatagramConnection)
    Connector.open("datagram://localhost:9001");
  try {
    byte[] payload = "Test Message".getBytes();
    Datagram datagram = 
      dgc.newDatagram(payload, payload.length);
    dgc.send(datagram);
  } finally {
      dgc.close();
  }
} catch (IOException x) {
  x.printStackTrace();
}

Next I show how an application might receive the datagram sent by the previous code snippet.

try {
  DatagramConnection dgc = (DatagramConnection) 
    Connector.open("datagram://:9001");
  try {
    int size = 100;
    Datagram datagram = dgc.newDatagram(size);
    dgc.receive(datagram);
    System.out.println(
      new String(datagram.getData()).trim());
  } finally {
      dgc.close();
  }
} catch (IOException x){
  x.printStackTrace();
}

In order to run the send and the receive examples, two instances of a MIDlet can be run from the Wireless Toolkit, one to perform the receive and one to perform the send.