It is ok to drink in public in most of Japan
In Tokyo and all of Japan, there are not many laws prohibiting the consumption of alcohol in public, whether it's on the sidewalk or in a park. Some parks, such as Shinjuku Gyoen park, prohibit, but if there's no signs posted, then it's fine. However, it's best to be considerate of others and not get too intoxicated. While law enforcement will not hassle you for drinking a beer on a street corner, they will talk to you if you become a public nuisance.