protected OffHeapStoredObjectAddressStack createFreeListForEmptySlot(
      AtomicReferenceArray<OffHeapStoredObjectAddressStack> freeLists, int idx) {
    return new OffHeapStoredObjectAddressStack();
  }
Tests override this method to simulate concurrent modification
geode-core/src/main/java/org/apache/geode/internal/offheap/FreeListManager.java