ConcurrentModificationException with LinkedHashMap

Question

Not sure what is triggering a java.util.ConcurrentModificationException when I iterate over the LinkedHashMap structure in the code below. Using the

Answer 1

In the constructor of LinkedHashMap you pass true to get the LRU behaviour (meaning the eviction policy is access order rather than false for insertion order).

So every time you call get(key) the underlying Map.Entry increments an access counter AND reorders the collection by moving the (last accessed) Map.Entry to the head of the list.

The iterator (implicitly created by the for loop) checks the modified flag, which is different from the copy it took originally, so throws the ConcurrentModificationException.

To avoid this you should use the entrySet() as the implementation is inherited from java.util.HashMap and therefore the iterator doesn't check the modification flags:

    for(Map.Entry e : lru_cache.entrySet()){
        System.out.println(e.getValue());
    }

Be aware this class isn't threadsafe so in concurrent environments you will need to use an potentially expensive guards like Collections.synchronizedMap(Map). In this scenario a better option might be Google's Guava Cache.