Weird error when sending DatagramPackets from a multicastSocket

Question

hi i got this error when running a program that uses a multicastSocket in different Threads
the error is java.lang.NullPointerException: null address || null buffer
        at java.net.TwoStacksPlainDatagramSocketImpl.send(Native Method)
        at java.net.DatagramSocket.send(DatagramSocket.java:638)
        at network.Main$LocalServer.runloop(Main.java:363)
        at network.Main$LocalServer.run(Main.java:382)line 363 is
                DatagramPacket dp=new DatagramPacket(b, 19);
                System.out.println("LocalServer: Packet created with 19 bytes");
                sleep(100);
                SL.send(dp);line 283 is (the run method of LocalServer)
public void run() {
            runloop();
        }variable SL is created at line 173 in class main.java
sl=new MulticastSocket(1234);
            System.out.println("local socket created");
            sa=new MulticastSocket(51234);
            System.out.println("global socket created");
            InetAddress g1=InetAddress.getByName("[ff05::1]");
            InetAddress g2=InetAddress.getByName("[ff1e::1]");
            sl.joinGroup(g1);
            System.out.println("local socket group joined");
            sl.setLoopbackMode(true);
            sa.joinGroup(g2);sl is passed to Thread as SL at line 290:
LocalServer s=new LocalServer(sl);
            s.start();and at line 343
SL=sl;i am using JDK 1.7.0
and Netbeans IDE 6.0
please help me fix this problem!
thanks in advance for any help

Answer 1

The DatagramPacket has to be constructed so as to contain the target address.
MulticastSocket.joinGroup() doesn't help that. It only affects incoming datagrams.