// simultaneous update for same key, some null, some non-null
    doParallelCacheOp(
        count,
        n -> {
          cache.asMap().compute(key, (k, v) -> n % 2 == 0 ? v + delimiter + n : null);
        });
    assertTrue(1 >= cache.size());
  }

  public void testCompute() {
    cache.put(key, "1");
    // simultaneous deletion
    doParallelCacheOp(
        count,
        n -> {