They aren’t a threat,right?

I heard of the Yakuza and those biker gang/greaser dudes.My question is if they are a threat to people.And if any of you ever encountered either one of them.

... nnnnnope.

not at all. Don't worry about it. They like to keep themselves underground and they wouldn't randomly attack a tourist... or anyone usually.
In my three semesters in Japan I've seen two different groups of men who were definitely yakuza. The first I got a picture with because my friend encouraged me to go with her...*sigh* and while they were saying rude things about what kind of poses we should do nothing happened except the picture.
The second time I just saw a few guys on the street, walked past them and that was all. Nothing happened.

So don't worry about it.

They might be a threat if you tried to set up a hostess bar on their turf. If you don't plan to do that, you will be fine. You don't see/hear the bikers much these days, they were more of a phenomenon back in the 70s/80s.

Well at least they are not like those gangs (you know,the Bloodz and the Cripts)in cities like L.A.,NYC,...or Detroit(shudder).Well Japan is a safe place with a yearly homicide rate of 0.62% (compare that to U.S.A with a rate of 68%!!)

Worst they'll ever do, is try to get you drunk and leave you with the tab.

Went to tokyo(near akihabara), a guy comes up speaking english (obvious yakuza), talks me into a bar, we get smashed, and he practically bolted for the door.

Wouldn't be surprised if it was a Yakuza owned bar either.