How to unit test a class can be used in a Set or Map (contains always works)

I discovered that some instance of a (java) class that I used in Maps and Sets sometimes fails to be found using the contains method.

The problem was simply that the class did not implement "hashCode" (but it has equals). And the subtlety was that it actually works most of the time: I only found out about the error when running some code with concurrency.

My problem now is that I'd like to make a unit test that assert that this class can be used reliably in a Set/Map (i.e that .contains works) but I cannot reproduce the bug.

As I say, almost all the time it actually works. I tried to make a Set of a couple of instances, then run multiple times contains with copies of these instances, in parallel. But it always works.

What test of mySet.contains(item-copy) would fails for sure if item class has not hashCode?

Here is the test pseudo code that don't fails when MyItem does not implements hashCode:

def testFindInSet(): Unit ={
  val items = Seq(---list of args---).map( args => new MyItem(args)}.toSet
  items.par.foreach{ item =>
    (1 to 100).par.foreach{ k =>
      val itemCopy = new MyItem(item)  // that's a copy constructor
      assertTrue("item not found", items(itemCopy))
    }
  }
}