Japan is probably the only non third world country which has yet to implement the concept of central heating. It does get fairly cold in the winter and it’s a common daily sight in offices across the country to see hordes of people shivering and complaining about the cold. The general thought on the matter is “It’s only three months, so you should be able to stick it out!”
There are forms of heating, albeit somewhat impractical. The first and most common would be your air conditioning unit. This does pump hot air around the room and it can get quite warm, as soon as one opens a door however or as soon as it gets switched off, the room gets instantly cold yet again. Kerosene heaters are probably the most usual form of heating, though they are problematic as they don’t heat the whole room, in fact the heat that radiates from them is quite confined; also you have to fill the buggers up with gasoline which is a pain.
Most houses have a kotatsu. These are basically floor tables with a heater underneath. Families tend to huddle together under the kotatsu as they also come with a blanket. Koatatsu tables are actually my favorite way to keep warm in the winter as they are cozy and not to heavy on electricity. These devices are for home use only though.
The best way to keep warm in the winter is simply to wear multiple layers of clothing, indoors and out. Some places of work literally have no heating. Schools tend to be the worst. I still have memories of teaching in large classrooms, the condensation steaming off of my breath and all the students wrapped up in blankets shivering.