@Override
    public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
        if (key == null) {
            return null;
        }
        return super.computeIfAbsent(key, mappingFunction);
    }
@Test
    public void testComputeIfAbsent() {
        Assert.assertEquals(null, map.computeIfAbsent(null, key -> ""));
        Assert.assertEquals(VALUE, map.computeIfAbsent(KEY, key -> ""));
        Assert.assertEquals(VALUE, map.get(KEY));
    }