Origin of Japanese
Japanese language (Nihongo) is primarily spoken in Japan (Nippon or Nihon) where it is national language.
According to History documents, before 3rd century AC, there’s spoken language in Japan but no writing system. Around 3rd century AC to 8th century AC, due to the trading between Japan and China, and the spread of Buddhism, Chinese writing system was imported to Japan. So, using Chinese writing system and Spoken Japanese constructed first and oldest known Japanese language.
Chinese Language: In Ancient Times, Chinese Language consists of pictures which represent their counterparts called pictograms or logograms. Ex: 1) Sun - A circle with lines around it. 2) Mountain - Continuous sharp ups and downs. Afterwards, these pictograms are simplified for writing and Ideograms etc. are introduced as the language evolved making these characters more than 60,000. Every character has its own meaning and sound. Nowadays, less than 10,000 characters are in use, which are called Hanzi.
Chinese Adaptation: When Japanese people adapted Chinese characters, they have also adapted that character original readings ,known as Onyomi (Chinese Readings) and given their own readings to characters based on that character meanings, known as Kunyomi(Japanese Readings). As the language evolved in both countries, reading and meanings on both sides went many changes and the Chinese adaptation in Japanese also continued, but some characters in Chinese have different readings and meanings than the already adapted Chinese characters. So, some are changed and some are added to already adapted characters.
Also, Chinese have more consonants and vowels than Japanese and many dialects, so more pronunciations. When adapting Chinese, Japanese people adjusted Onyomi to their pronunciations.
Ex: æ¼¢å? Hanzi (Han, Zi) - Chinese name for characters and Kanji (Kan, Ji) – Japanese name for characters. Written same in both languages but pronounced differently.
To increase literacy, Chinese people simplified Hanzi characters which are called Simplified Chinese while the original characters are called Traditional Chinese. Japanese also created few characters from already existed characters solely used in Japanese, and these characters have only Kunyomi and no Onyomi. Kanji of Modern Japanese consists of some Simplified and many Traditional Chinese characters.
Conclusion: Even though Chinese and Japanese have some origin between them, nowadays other than Hanzi/Kanji , some words and their meanings, everything else is different.