I have read somewhere that in Japan, it is rude to eat anything while walking in public. However, is it also rude to simple be eating something while standing out on a street corner?
Most Japanese people won’t do that but there are exceptions all around. Besides, Japanese won’t judge visitors/foreigners. So, you should refrain from eating in these places but a quick bite while waiting for buses/trains won’t hurt.
It's not well-mannered but neither is it unforgivable. However, if you say "standing out on a street corner" I'd say that at least you should stand somewhere with less traffic and movement. Streets can be crowded and nobody wants to get food or drink spilled on them if they bump into you, and it will be seen as much ruder if you are doing so in a crowded place.
In order of preference:
- Find somewhere to sit down to eat, even if it is a park bench.
- Find somewhere with less traffic, like an empty alcove, or stand near a wall where people won't have to avoid you.
And as a general rule, if it's a larger meal or it requires utensils to eat, you should be sitting down. The above mostly applies to small snacks you can eat with your hands (e.g. rice balls, chicken pieces, etc.).