How do you write a test that actually creates contention?

Threads started in a loop tend to run one after another, because starting a thread takes longer than the body of a small test. A CyclicBarrier or a CountDownLatch used as a starting gate lines them up so they collide:

int threads = 8;
var start = new CountDownLatch(1);
var done  = new CountDownLatch(threads);
for (int i = 0; i < threads; i++) {
    new Thread(() -> {
        start.await();              // everyone waits here (try/catch omitted)
        for (int n = 0; n < 10_000; n++) counter.increment();
        done.countDown();
    }).start();
}
start.countDown();                  // released together
assertTrue(done.await(10, SECONDS));
assertEquals(threads * 10_000, counter.get());

Then say the caveat, because it is the interesting half: the barrier is itself a memory barrier, so this catches lost updates but hides visibility bugs.