java - Adding and removing elements to a Collection -

i'm new java , i'm making game player has eat cookies. these cookies elements of arraylist. arraylist modified 2 threads : -one iterates on , removes cookies have been eaten, using iterator.remove() -one adds cookie arraylist every 5 seconds

sometimes concurrentmodificationexception, , know it's because behavior of iterator.remove() "unspecified if underlying collection modified in other way while iteration in progress", stated in java tutorial sun. how should proceed ?

edit : updated code with

list<cupcake> cake = collections.synchronizedlist(new arraylist<cupcake>()); 

here's spawner :

public class cupcakespawner extends thread {     private background back;      public cupcakespawner(background back) {         this.back = back;     }      public void run() {         while(true) {             if(back.getcake().size() < 15)                 back.getcake().add(new cupcake());             try {                 thread.sleep(5000);             } catch (interruptedexception e) {                 e.printstacktrace();             }         }     } } 

my update method :

public void update() {     list<cupcake> cake = back.getcake();     iterator<cupcake> itrc = cake.iterator();     while(itrc.hasnext()) {         cupcake cupcake = (cupcake)(itrc.next());         checkcollisioncup(cupcake);         if(cupcake.iseaten())                  itrc.remove();         }     } } 

arraylist not threadsafe implementation of list.

use copyonwritearraylist instead - not cause explosion if add elements while iterating.