@Override public long putIfAbsent(long key, long value) {
        assert value != nullValue : "putIfAbsent() called with null-sentinel value " + nullValue;
        SlotAssignmentResult slot = hsa.ensure(key);
        if (slot.isNew()) {
            mem.putLong(slot.address(), value);
            return nullValue;
        } else {
            return mem.getLong(slot.address());
        }
    }
@Test(expected = AssertionError.class)
    @RequireAssertEnabled
    public void test_putIfAbsent_invalidValue() {
        map.putIfAbsent(newKey(), MISSING_VALUE);
    }