Handling exhaustion in spliterator implementations is error-prone, especially when two fields are involved, and fields transition from null to non-null back to null, sometimes in the same operation.

Here's a test that finds 3 implementations that get it wrong:

    /**
     * Concurrent Spliterators, once exhausted, stay exhausted.
     */
    public void testStickySpliteratorExhaustion() throws Throwable {
        if (!impl.isConcurrent()) return;
        if (!testImplementationDetails) return;
        final ThreadLocalRandom rnd = ThreadLocalRandom.current();
        final Consumer alwaysThrows = e -> { throw new AssertionError(); };
        final Collection c = impl.emptyCollection();
        final Spliterator s = c.spliterator();
        if (rnd.nextBoolean()) {
            assertFalse(s.tryAdvance(alwaysThrows));
        } else {
            s.forEachRemaining(alwaysThrows);
        }
        final Object one = impl.makeElement(1);
        // Spliterator should not notice added element
        c.add(one);
        if (rnd.nextBoolean()) {
            assertFalse(s.tryAdvance(alwaysThrows));
        } else {
            s.forEachRemaining(alwaysThrows);
        }
    }

In LTQ and CLQ, constructions of the form

            if (!exhausted &&
                ((p = current) != null || (p = firstDataNode()) != null) &&

fail to set exhausted to true when firstDataNode returns null.

Here's idiomatic code to get this right:

        private void setCurrent(Node p) {
            if ((current = p) == null)
                exhausted = true;
        }

        private Node current() {
            Node p;
            if ((p = current) == null && !exhausted)
                setCurrent(p = firstDataNode());
            return p;
        }
...
            Node p;
            if ((p = current()) != null) {


In PriorityBlockingQueue, the lazy init code for forEachRemaining does not call getFence (why?), and fails to set array, so a subsequent call to forEachRemaining might return more elements!

        public void forEachRemaining(Consumer<? super E> action) {
            Object[] a; int i, hi; // hoist accesses and checks from loop
            if (action == null)
                throw new NullPointerException();
            if ((a = array) == null)
                fence = (a = toArray()).length;