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POSynchronized threads throwing a CurrentModificationException
 

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    <p>I am having trouble getting the concurrency running on a simple UDP chat client I am writing, it all works perfectly until I try to add a new contact to the outgoing message list stored in Peer, it throws a CurrentModificationException, can someone help me understand where I have gone wrong? </p> <p>Here are my classes</p> <pre><code> import java.net.*; import java.util.*; import java.io.*; public class Chatter { public static class ReceiveMess extends Thread{ DatagramSocket ds; public ReceiveMess(DatagramSocket s){ ds = s; } byte[] Rbuf = new byte[1000]; DatagramPacket Rdgp = new DatagramPacket(Rbuf, Rbuf.length); public synchronized void run() { try{ while (true){ for(Peer p : Peer.PeerList){ ds.receive(Rdgp); String rcvd = new String(Rdgp.getData(), 0, Rdgp.getLength()) + ", from address: " + Rdgp.getAddress() + ", port: " + Rdgp.getPort(); System.out.println(rcvd); } } } catch(IOException e) { System.out.println(e); } } } public static class SendMess extends Thread{ DatagramSocket ds; public SendMess(DatagramSocket s){ ds = s; } int SPORT = 40080; byte[] Sbuf = new byte[1000]; DatagramPacket Sdgp = new DatagramPacket(Sbuf, Sbuf.length); public synchronized void run() { try{ while (true) { BufferedReader consR = new BufferedReader(new InputStreamReader(System.in)); String MessOut = consR.readLine(); if(MessOut.startsWith("/NEW")){ try{ String[] splitArray = MessOut.split(" "); String newIP = (splitArray[1]); Peer p = new Peer(newIP); System.out.println(newIP + " added to the contacts list"); continue; } catch(Exception e){ System.out.println("Please format NEW IP address's as NEW XXX.XXX.XXX.XXX"); continue; } } else{ Sbuf = ("Server Said: " + MessOut).getBytes(); for(Peer p : Peer.PeerList){ DatagramPacket out = new DatagramPacket(Sbuf, Sbuf.length, p.IP, SPORT); ds.send(out);} } } } catch(IOException e) { System.out.println(e); } } } public static void main(String [] args){ try{ for(String s : args){ String address = s; Peer peer = new Peer(address); } int PORT = 40080; DatagramSocket ds = new DatagramSocket(PORT); Peer.PrintList(); SendMess sendmess = new SendMess(ds); sendmess.start(); ReceiveMess receivemess = new ReceiveMess(ds); receivemess.start(); } catch(Exception e){ System.out.println(e); } } } And my peer class, import java.net.*; import java.util.*; public class Peer{ InetAddress IP; static List&lt;Peer&gt; PeerList = new LinkedList&lt;Peer&gt;(); Peer(String clientAddress){ try{ IP = IP.getByName(clientAddress); AddToList(this); } catch(UnknownHostException e){ System.out.println(e.toString()); } } public synchronized void AddToList(Peer peer){ PeerList.add(this); } public List&lt;Peer&gt; GetList(){ return PeerList; } public static void PrintList(){ for(Peer p : PeerList){ System.out.println(p.IP.toString()); } } } </code></pre>
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    1. USJames Luxton
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      1. VTUpMod
    1. COIn which line of your code you get the Exception? please add the stacktrace
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      1. USDror Bereznitsky
    2. COYou need to synchronize on a shared object, otherwise it doesn't really do anything. It is considered bad practice to extends Thread and using synchronized in it is a good way to see some tricky puzzlers ;)
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      1. POSynchronized threads throwing a CurrentModificationException
      1. USPeter Lawrey
    3. COI receive the error on line 27 which is the for each loop in ReceiveMess, I understand the error is iterating through the list while another thread is doing the same but I cannot fix it
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      1. USJames Luxton
 

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