java - ConcurrentModificationException without modifying object -

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i've got following piece of code causing concurrentmodificationexception. i'm not modifying products object @ all, 

list<product> products = product.findactivebyfilter(filters); set<long> temp = new hashset<long>(); list<product> resultsettemp = new arraylist<product>(); (product product : products) { // << exception points     if(!temp.contains(product.getid())){         temp.add(product.getid());         resultsettemp.add(product);     } } products.clear(); products.addall(resultsettemp);

i've seen exception pop-up several times, cannot reproduce (it happens randomly).

product.findactivebyfilter method returns new instance of list<product> has been build cached list<product>.

edit: i've found way reproduce error. code called when client wants products (its webshop), , website loads more items when client scrolls down. triggers exception (as server not-yet done responding products, , gets call it). race conditions, fun!

as said already, caused seperate tread 'modifying' products (as cached instance). i've changed implementation of product.findactivebyfilter return new arraylist<product>(products); instead of reference cached value, when no filters applied (thus no filtered result given back).

public static list<product> findactivebyfilter(arraylist<filterpair> filters) {     list<product> products = getcachedallproductsbyfirstsupplier();      if (products == null) {         products = new arraylist<product>();     }      if(filters.size() > 0) {         list<product> productsfiltered = new arraylist<product>(products);         // ... many checks here filters ...          return productsfiltered;     }      return new arraylist<product>(products); // not give cached copy, 'return products;' }

there 2 calls findactivebyfilter, called website. first 1 did include filter, second 1 did not (so first 1 still busy, while second 1 returned directly).


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