Please help with setData of Datagram class

Question

hi
im new to J2ME stuff. while working with BOOK "wireless java programming with java 2 micro edition" i encountered a problem in networking chapter.
Im gating NULLPointer Exception while using setData() method of Datagram Class
               byte[] message = new byte[length];
               System.arraycopy(msg.getBytes(),0,message,0,length);
// null poninter exception// sendDatagram.setData(message,0,length);
               sendDatagram.setLength(length);
the whole code of client program is as...
import javax.microedition.midlet.*;
import javax.microedition.io.*;
import javax.microedition.lcdui.*;
import java.io.*;
public class DatagramClient extends MIDlet implements CommandListener{
     private Form mainScreen;
     private TextField sendingField;
     private Display myDisplay = null;
     private DatagramClientHandler client;
     //define the GUI components for displaying the returned message
     private Form resultScreen;
     private StringItem resultField;
     private String resultString;
     Command sendCommand = new Command("SEND",Command.OK,1);
     public DatagramClient() {
          //intialize the GUI components
          myDisplay = Display.getDisplay(this);
          mainScreen = new Form("Datagram Client");
          sendingField = new TextField("Enter your message",null,30,TextField.ANY);
          mainScreen.append(sendingField);
          mainScreen.addCommand(sendCommand);
          mainScreen.setCommandListener(this);
     public void startApp(){
          myDisplay.setCurrent(mainScreen);
          client = new DatagramClientHandler();
     public void pauseApp(){
     public void destroyApp(boolean unconditional){
     public void commandAction (Command c, Displayable s) {
          if(c==sendCommand){
               //get the message text from user input
               String sendMessage = sendingField.getString();
               try{
                    resultString = client.send_receive(sendMessage);
               }catch(IOException e){
                    System.out.println("Failed in Send_receive(); "+e);
               resultScreen = new Form ("Message confirmed:");
               resultField = new StringItem(null,resultString);
               resultScreen.append(resultField);
               resultScreen.setCommandListener(this);
               myDisplay.setCurrent(resultScreen);
     public class DatagramClientHandler extends Object {
          private DatagramConnection dc;
          private Datagram sendDatagram;
          private Datagram receiveDatagram;
          public DatagramClientHandler() {
               try{
                    dc = (DatagramConnection)Connector.open("datagram://"+":9000");
                    receiveDatagram = dc.newDatagram(dc.getMaximumLength());
               } catch(IOException e){
                    System.out.println(e.toString());
          public String send_receive(String msg) throws IOException {
               int length = msg.length();
               System.out.println("length of string "+length+"message "+msg);
               byte[] message = new byte[length];
               System.arraycopy(msg.getBytes(),0,message,0,length);
               System.out.println("message is : "+message+" length is "+length);
               sendDatagram.setData(message,0,length);
               sendDatagram.setLength(length);
               String retval = "";
               try{
                    dc.send(sendDatagram);
                    dc.receive(receiveDatagram);
                    byte[] data = receiveDatagram.getData();
                    retval = new String(data,0,receiveDatagram.getLength());
               }catch (Exception e ){System.out.println("sirfra excep: "+e);}
               finally{
                    if(dc!=null) dc.close();
               return retval;
sir i hav to submit an assignment on networking and RMS on thursday. so please help me as early as possible . ill be really very thankful to you.

Answer 1

please change "sendDatagram" to "receiveDatagram"
public String send_receive(String msg) throws IOException {
int length = msg.length();
System.out.println("length of string "+length+"message "+msg);
byte[] message = new byte[length];
System.arraycopy(msg.getBytes(),0,message,0,length);
System.out.println("message is : "+message+" length is "+length);
sendDatagram.setData(message,0,length);
sendDatagram.setLength(length);