I had the misfortune of spending 2 weeks, courtesy of the Japanese Police, in a Japanese jail in Yokohama in 1987 for posession of a minute amount of hashish. It's quite a long story and I will post it shortly in a couple of parts.
Let me tell you one thing, it is not a pleasent experience. Once arrested you have absolutely no rights whatsoever. You are guilty until proven innocent, you cannot make a phone call or let anyone know where you are until you are indicted, which can be up 30 days, you cannot post bail until indicted or you are realeased. You cannot even have a lawyer present at questioning!
I was not abused by either the police, detectives, or my cell mate, nor did they treat me harshly as long as you followed the rules. The questioning is intense and may be a couple of times a day for a minimum of an hour at a time.
When I was indicted I was allowed to post bail. 3,000,000yen (US$30,000) at 10%. My lawyer cost me about 300,000yen (US$3,000) Trial was some 7 months later. If you can't post bail of course you will spend your time in jail until the trial.
You will go to court if indicted and will face one judge or, in my case, a panel of three judges. If convicted you may receive prison time or probation. It all depends on how contrite you are. If you are a single foreigner with no relations in Japan you will be deported and may be banned from Japan for at least 5-10 years. In my case, I had a Japanese wife and was not deported or banned although I could have been. I guess my sincere, honest apology convinced the judges.
Prison I can't say as I never went. I received 3 years probation. The jail on the other hand was not bad from what I heard in the states. 3 hots a day. Up at 6am in bed by 9pm. Allowed out of cell 3 times/day for smoking and stretching. Very, very boring. No abuse from the guards whatsoever. It was not a pleasent experience and is one I will never experience again.
In the end you don't want to experience it. Trust me.