executorservice - java.util.concurrent.ExecutionException: java.util.ConcurrentModificationException -

i getting concurrent modification exception when executing following code:

mymap global variable , hashmap

 callable<string> task = new callable<string>() {    @override    public string call() {                  mymap.put("myid", "id2");       mymap.put("myname", "joe");       string id = mymap.get("myid");       system.out.println("id is: "+ id+ ", mymap before: "+mymap.tostring());       mymap.remove("myid");       system.out.println("id is: "+ id+ ", mymap after: "+mymap.tostring());       return id;    }  };   list<callable<string>> tasks = collections.ncopies(7, task);  executorservice executorservice = executors.newfixedthreadpool(7);  list<future<string>> futures = executorservice.invokeall(tasks);  list<string> resultlist = new arraylist<string>(futures.size());   (future<string> future: futures){     resultlist.add(future.get());  } 

the exception thrown lines:

resultlist.add(future.get()); 

and

system.out.println("id is: "+ id+ ", mymap after: "+mymap.tostring());  

however if try

system.out.println("srcnode after: "+srcnode.tostring()+ ", id: "+id); 

instead error seems disappear. clues on whats going on?

since there multiple threads modifying same instance - error. regarding,

why 1 point statement works , not other

threads not guaranteed provide same outcome every time. possible if execute program many times, different output (no exception, exception @ different line etc).

example - executed program 3 times, execution successful 2 times , got concurrent exception third time.

to conclude, 1 cannot guarantee order or timing of execution of threads. in order avoid these errors use synchronization or hashtable thread safe - comes @ cost of performance. also, having mymap declared local variable method work - since each thread have own copy of local variable - assuming instance variable reason.