if you are very hungry and having a few money in Japan, you should run into KONBINI (convenience store) and buy one "ONIGIRI".
To buy "ONIGIRI", just bring it to register counter and say "ONEGAI SHIMASU" meaning "please". It will cost you 80 to 200 JPY.
After buying it, you may eat it during walking around. Though Japanese will not eat something during walk, "ONIGIRI" would be allowed.