According to the Consulate General of the People's Republic of China in Gothenburg:
Foreigners wishing to travel to China generally need to apply for a visa in advance. However, ordinary passport holders from Singapore, Brunei and Japan can gain access to China for up to 15 days without applying for a visa for business purposes, sightseeing, relatives and friends, or transit. You can enter and leave the ports open to foreigners.
Swedish citizens are not on the list of citizens who have visa-free access to China. So you need a visa to visit mainland China.
However, you can also visit Hong Kong without a visa:
Ordinary passport holders from Sweden can travel to Hong Kong for up to 90 days without a visa application for business purposes, sightseeing, visiting relatives and friends, or transit.
I recommend you to read this Wikipedia article If you want to know which countries you can visit without a visa. It is generally a good source of information. howeverAlso check with an official source before you actually travel to a country to make sure that you can really travel there without a visa. Official sources include: embassies, consulates, updated government websites (preferably official websites of the country you are traveling to), etc.