ConcurrentModificationException while iterating Map [duplicate]

Answer 1

You cannot remove an element from a collection while iterating it unless you use an Iterator.

This is what's causing the exception.

buyingItemEnumerationMap.remove(item.getKey());

Use Iterator#remove() to remove an element while iterating over your collection like

Iterator> iterator = 
                           buyingItemEnumerationMap.entrySet().iterator();
while (iterator.hasNext()) {
   Map.Entry item = iterator.next();
   if(RandomEngine.boolChance(50)){ //will delete?
      iterator.remove();
   }
   //..
}

EDIT : (in response to OP's comment)
Yes, the deletions done through Iterator#remove() over the Set returned by HashMap.entrySet() would reflect in the underlying Map as the Set is backed by it. Quoting the JavaDoc here:

Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.