Japan is a country that is mostly rugged and mountainous with 29,751 km of coastline with no land boundries.
https://www.cia.gov/library/publications/the-world-factbook/geos/ja.html
Map indicates the average temperature in Japan which varies from tropical in south to cool temperate in north.
Japan is primarily made up of Japaneses nationals further showing that it is an island onto its own. Percentanges found from the CIA World Factbook.