der Japaner
Japanese man
A male person who comes from Japan or has Japanese citizenship.
Der neue Austauschstudent in unserer Klasse ist Japaner.
The new exchange student in our class is Japanese.

Japanese man / adjustable wrench
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Japanese man
A male person who comes from Japan or has Japanese citizenship.
Der neue Austauschstudent in unserer Klasse ist Japaner.
The new exchange student in our class is Japanese.

The ending '-er' is a typical way to form the masculine term for an inhabitant (e.g. from the country 'Japan' comes 'der Japaner' (the Japanese man)). The corresponding adjective is 'japanisch' (Japanese).
Mein neuer Kollege ist Japaner und spricht perfekt Deutsch.
My new colleague is Japanese and speaks perfect German.
To distinguish the two meanings, pay attention to the context. If it is about people, travel, or culture, it refers to the person. If it is about tools or repairs, it could refer to the wrench.
Der Japaner, den ich gestern kennengelernt habe, studiert in Berlin.
The Japanese man I met yesterday is studying in Berlin.
The term 'Japaner' (adjustable wrench) for a wrench is colloquial and is considered outdated today. You might still hear it from older people in a workshop.
Mein Opa hat in seiner Werkzeugkiste noch einen alten Japaner.
My grandpa still has an old adjustable wrench in his toolbox.
Japanese
'japanisch' and 'der Japaner' are both connected to Japan, but they have different grammatical roles. 'japanisch' is an adjective and describes things, culture, products, or the language. For example, you say 'japanisches Essen' (Japanese food) or 'auf Japanisch' (in Japanese). 'der Japaner' is a noun and means a male person from Japan or with Japanese citizenship. The words can be confused because they look similar and refer to the same country. The important point is: for characteristics and language, use 'japanisch'; for a person, say 'Japaner' or, accordingly, 'Japanerin'.

Der Japaner erklärt den Gästen geduldig den Weg zum Bahnhof.
The Japanese man patiently explains the way to the station to the guests.
Im Restaurant bestellt Lena eine japanische Suppe.
At the restaurant, Lena orders a Japanese soup.