java - ConcurrentModificationException Issue When Running Two Thread -

so, working on server has support multie clients, have 1 thread checks if sockets have connected given port , adds them arraylist other thread uses update need client (update info, check datainputstream, send text on server) , on.

client code:

public class loop implements runnable{  arraylist<clientinstance> clientsconnected = new arraylist<clientinstance>();  @override public void run() {     while(true) {         checkinputstream();     }  }  public void checkinputstream() {     (clientinstance s : clientsconnected) {         s.checkinputstream();     } } 

server code:

public synchronized void waitforclient() {     try {         system.out.println("waiting client on port: "                 + serversocket.getlocalport());         socket client = serversocket.accept();         system.out.println("client connected! " + client.getinetaddress());         loop.getclientsconnected().add(new clientinstance(client));         system.out.println("client added clients connected! ");     } catch (ioexception e) {         e.printstacktrace();     } } 

but when run server , connect 1 client it works fine, when connect 1 gives me issue:

exception in thread "thread-1" java.util.concurrentmodificationexception @ java.util.arraylist$itr.checkforcomodification(unknown source) 

what do?

this because modifying arraylist (i.e. adding element in list in waitforclient() method ) , @ same time iterating in checkinputstream() method.

as mentioned @arjit use copyonwritearraylist instead of arraylist.