@Turkeyjerkey Sure, I don't refute that I have 0 data or personal experience to back up how frequent it is; but doesn't change that it's a serious issue not to be taken lightly and Japan is notoriously known for it.
Think for a second of how difficult a position people are placed when they are subjected to such a crime - I'm willing to bet a large proportion of such events go unreported because an infinite number of factors ranging from mental age (An inexperienced 12 year old vs a mature 30 year old), personality (Shy, quiet, introverted people), not wanting to trouble others (Cultural/social habits), just bearing with it (Nobody to talk to, nobody would listen), etc.
I would then question what do you even mean by common? 10 reported cases while 10 go unreported (It's practically impossible to figure out how many actually cases go unreported in the first place; statistics can only guesstimate so much - some of the more radical estimates apparently put this figure at 100,000 cases per year)? 100? 1000? At what population ratio, 2000 reported cases per 127,000,000 people (Japan population) vs 2000 reported cases per 10,000,000 people (Tokyo population)? At some point it's meaningless trying to quantify your definition as there's no fair and simple way of applying it, but it doesn't change my original point that Japan is notorious for chikans and that it does occur/has happened at least once in the past.
@BigClover sure, some people have that kind of self restraint; but not everyone does. In an ideal world, everyone would be clever, good natured, and the model citizen™; but neither reality nor fiction is so kind