Here is a method which will send a message to a quake server. The message type can be either a control message or a game message.

It returns a byte array containing the server response. If the 50 ms timeout elapses, then it returns a byte array of length 0.

    static final int CONTROLMSG=0;
    static final int GAMEMSG=1;

    int clientPktNum=0;
    int serverPktNum=0;

    byte[] sendPacket(int type, byte[] command)
    {
        ByteArrayOutputStream bOut = new ByteArrayOutputStream();
        byte[] message;
        if(type == CONTROLMSG)
        {
            bOut.write(CTRLheader, 0, CTRLheader.length);
            bOut.write(command, 0, command.length);
            message = bOut.toByteArray();
        }
        else //GAME MESSAGE
        {
            //assemble packet numbers
            String hexCode;
            int cNumber;
            byte [] cPktNumber = {(byte)0, (byte)0, (byte)0, (byte)0};
            byte [] sPktNumber = {(byte)0, (byte)0, (byte)0, (byte)0};
            //check toggle bit
            if(bit31)
            {
                cNumber = clientPktNum | 0x80000000;
                bit31=false;
            }
            else
                cNumber = clientPktNum;
            //little-ending (reverse byte order)
            cPktNumber[3] = (byte)((cNumber >>> 24) & 0xFF);
            cPktNumber[2] = (byte)((cNumber >>> 16) & 0xFF);
            cPktNumber[1] = (byte)((cNumber >>> 8) & 0xFF);
            cPktNumber[0] = (byte)((cNumber >>> 0) & 0xFF);

            sPktNumber[3] = (byte)((serverPktNum >>> 24) & 0xFF);
            sPktNumber[2] = (byte)((serverPktNum >>> 16) & 0xFF);
            sPktNumber[1] = (byte)((serverPktNum >>> 8) & 0xFF);
            sPktNumber[0] = (byte)((serverPktNum >>> 0) & 0xFF);

            bOut.write(cPktNumber, 0, cPktNumber.length);
            bOut.write(sPktNumber, 0, sPktNumber.length);
            bOut.write(command, 0, command.length);
            message = bOut.toByteArray();
            clientPktNum++;
        }

        DatagramPacket dpQuery = new DatagramPacket(message, message.length,
            serverAddress, port);
        byte[] serverResponse= new byte[1024]; //set a default length
        DatagramPacket dpAnswer = new DatagramPacket(serverResponse,
                serverResponse.length);
        DatagramSocket dSocket = null;
        boolean packetNotReceived = false;
        try
        {   
            dSocket = new DatagramSocket();
            dSocket.setSoTimeout(50);
            dSocket.send(dpQuery);
            dSocket.receive(dpAnswer);
        }   
        catch (InterruptedIOException iioe) 
        {   iioe.printStackTrace();
            //handle socket timeout
            packetNotReceived = true;
        }
        catch (SocketException se) 
        {   se.printStackTrace();
            packetNotReceived = true;
        }
        catch (IOException ioe) 
        {   System.out.println("can't send packet!"); 
            ioe.printStackTrace();
            packetNotReceived = true;
        }
        finally
        {
            if(dSocket!=null)
                dSocket.close();
        }
        int responseLength = dpAnswer.getLength();
        byte[] result=new byte[0];
        if(!packetNotReceived)
        {
            result = new byte[responseLength];
            for(int i=0; i < responseLength; i++)
                result[i]= serverResponse[i];
            if(type == GAMEMSG)
                serverPktNum++;
        }
        return result;
    }