Japanese culture has some overlap with Chinese culture, but they existed separately long before the cultural exchange.
And the funny thing about the Japanese writing system is that they tried to write Japanese in pure Chinese characters but failed, so instead they invented kana and ended up with three different alphabets.
Still, the Kanji has become so uniquely Japanese that Chinese and Japanese are generally unintelligible between speakers who know the same characters because they often mean different things in their own language.