public Printed<K, V> withKeyValueMapper(final KeyValueMapper<? super K, ? super V, String> mapper) {
        Objects.requireNonNull(mapper, "mapper can't be null");
        this.mapper = mapper;
        return this;
    }
@Test
    public void shouldThrowNullPointerExceptionIfMapperIsNull() {
        assertThrows(NullPointerException.class, () -> sysOutPrinter.withKeyValueMapper(null));
    }