^ Your mac does not need an adapter to work in Japan. The big square block *is* the power adapter, and the plugs in america are the same as in Japan for two pronged plugs. Three pronged american plugs (with the bottom prong that is the grounder) will not fit in Japanese plugs. Therefore, all PC users should find something that changes their three pronged charger plug into a two pronged. Mac users will be immune to this. If macs need an extention that works in Japan, during orientation if you can run to Ginza to the apple store, not only do all the staff speak fluent english (blew my mind when I went in) but you can get an extension cord that has a two pronged plug and a little green grounding wire that comes out of it. I've got one of these if anyone wants me to take a picture to see what it looks like, but basically I don't plug in the grounding wire ever so it just chills there when I use the plug.
Electronics from the US will also work fine in Japan barring that you do not try to bring an alarm clock, or any kind of plug-in clock, with you. Flat irons, hair curlers, anything else like that, work fine. Japan is just one or two volts below america so the difference is insubstantial. You'll probably just be finding yourself turning the temperature up on things like hair styling items to make up for the slight difference. And again, you don't want to bring clocks because they will run slower and keep time badly due to the tiny voltage difference.
When you get a cell phone there, you can use it as an alarm clock. Or, if you want to bring along your american phone and use it as an alarm clock till you get your Japanese one that's useful too. Otherwise, american electronics that have two pronged plugs should work perfectly fine there.
My luggage advice: when I first did study abroad, I had two giant pieces of luggage that only had one set of wheels and I was overweight on both of them. While I feel that being overweight on your luggage is going to be absolutely impossible to avoid going to Japan (because you're going to be there for at least a year!), I really really really wished that I'd had the new four wheel luggage bags. When I came back home, I did end up going to sams/costco and buying a luggage set that was not only hard covered but had the four wheels. Let me say now that if you know you're going to be overweight on large luggage going through Narita airport, you WANT the four wheels because struggling through customs wielding both bags solo on two wheels each (especially if you're tiny like me) is going to kill you after 24+ of travel. I also destroyed the wheels on my original luggage from study abroad because of rough handling at the airport and the heavy load, so investing in new luggage at a good price at places like sams and costco will be worth it I think.
Speaking of overweight luggage, the charge leaving the US is $50, but for the same thing leaving japan it was only $17, so.....hahaha. I would say go ahead and pack everything you need even if it's overweight knowing that it probably won't be as bad coming home again.
Also, who is considering bringing study books with them? Like Japanese books or dictionaries and such? I'm feeling really torn about this, and I don't know if I'm comfortable mailing them to myself because JPP tore up the last box of books I sent and lost over half of them and gave me someone else's crappy '70s fantasy books instead.