Usually native speakers would prefer the adjective rather the noun in these cases ("I am Japanese", "I am American"), or the prepositional phrases ("I am from Japan", "I am from the U.S.").

Where we don't have a subject pronoun to which the adjective can be attributed, we would usually supply "person" or another noun referring to a kind of person ("A Japanese person told me...", "Any Chinese person would readily recognize the meaning of hundreds of kanji based on the influence of hanzi", "My friend married a Japanese woman", "My mother goes to a Chinese acupuncturist").