Japanese vs Asian

Japanese and Asian both are nouns.

Word	Noun	Adjective	Verb	Adverb
Japanese	Yes	Yes	No	No
Asian	Yes	Yes	No	No

As nouns, Asian is a hypernym of Japanese; that is, Asian is a word with a broader meaning than Japanese:

Other hypernyms of Japanese include Asiatic.

Japanese (noun)	Asian (noun)
the language (usually considered to be Altaic) spoken by the Japanese	a native or inhabitant of Asia
a native or inhabitant of Japan

Japanese (adjective)	Asian (adjective)
of or relating to or characteristic of Japan or its people or their culture or language	of or relating to or characteristic of Asia or the peoples of Asia or their languages or culture