ConcurrentModificationException thrown by HashMap.compute() operating on an empty Map, expected/unexpected?

Patrick Zhang OS patrick at
Sat Mar 30 05:34:26 UTC 2019

Here I have a case simplified from a practical issue that throws ConcurrentModificationException (CME) unexpectedly (I think). [0] creates a HashMap, keeps it empty, and calls m.computeIfAbsent() or m.compute(), in which a "sneaky" m.clear() occurs, some of the test cases throw CME although there were no "structural" changes in fact. (A structural modification is defined as "any operation that adds or deletes one or more mappings...").

This case cannot be reproduced with jdk8u, while jdk9 and beyond can, after the bug [1] got fixed for computeIfAbsent() concurrent co-modification issues. A couple of test cases [2] were introduced at that time, and the focus was to verify the behaviors at resizing, while empty maps were not tested.

A possible "fix" for this issue is to move the unconditional "modCount++" [3] into the if-clause, which indicates that a "structural" change would be happening indeed.

public void clear() {
    Node<K,V>[] tab;
-   modCount++;
    if ((tab = table) != null && size > 0) {
+        modCount++;
          size = 0;
          for (int i = 0; i < tab.length; ++i)
            tab[i] = null;

Therefore, a dilemma here is "modCount++ before-if-clause but overkills some cases" vs. "modCount++ into-if-clause but weakens the CME checking potentially". I want to make balance regarding how to "throw CME on a best-effort basis" more appropriately. Any suggestion?

I understand that CME here in cannot guarantee much and may be only for debugging purpose, any concurrent modification needs to be typically accomplished by synchronizing on some object that naturally encapsulates the map. So the mentioned issue is a just a tricky case.



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