Most countries have longer official names (eg, Principality of Andorra, Commonwealth of Australia, or Argentine Republic). Japan is... just Japan (in English, at least). How'd they end up with such a short name?
(My gut tells me that it has something to do with the US occupation but my gut is very often wrong.)
I'm aware that there are other states with such straightforward official names - Canada, Georgia, Greece, Ireland, Malaysia, to name a few - but it seems that there are at most about a dozen. I'm also curious about those countries, if there's an interesting story.
Your gut instinct was right- before the 1947 constitution, Japan had the much longer official name "Empire of Great Japan" 大日本帝國 Dai Nihon Teikoku, compared to its shorter present form of "State of Japan" 日本國 Nihonkoku or even just "Japan" 日本 Nihon/Nippon. However the longer state name of the imperial era was modeled on Western practices, implemented alongside the Meiji reforms, rather than the historical customs of Japan and other East Asian countries.
The general pattern for states within the Chinese cultural sphere (itself a term first coined in Japanese historiography, as the "Chinese character cultural sphere" 漢字文化圈) has been state names composed of one or two Chinese characters, without any equivalent to western terms for feudal ranks (county/duchy/principality/kingdom) or political forms (republic/commonwealth/empire). Chinese regimes took one-character names, sometimes with an added "Great" (漢 Han, 唐 Tang, 大清 Da Qing) and the land itself was called mostly by two-character forms like Zhonghua 中華, Shenzhou 神州 or 中国 Zhongguo. Korean state names included Goryeo 高麗, Joseon 朝鮮, and the modern Hanguk 韓國; Vietnam 越南 previously went by Dai Viet 大越 or An Nam 安南. Japan, for its part, was first known by the slightly-pejorative name Wakoku (倭國, meaning something like "country of kneelers") given to them by the Chinese, and later as Daiwa/Yamato 大和 or, since at least the 8th century, the modern Nihon/Nippon 日本. Further elaborations, like the People's Republic of China 中華人民共和國, Republic of Korea 大韓民國, or Socialist Republic of Vietnam 越南社會主義共和國 all reflect the intrusion of Western political ideology from the 19th century onwards.
During the period of intense Westernization after the Meiji Restoration, Japan also took on a Western-style state name designating itself as an imperial regime. Following World War 2, Japan could hardly be considered a "republic" given the continued existence of the imperial household, but the term "empire" was also unacceptable given the connotations of militarism and colonialism. This led Japan to revert to the traditional-sounding yet ideologically-vacant "State of Japan" 日本國.