If you highlight all or part of multiple lines of code and press Ctrl-/, then if the code is not commented it out, then the editor will comment it out by prefixing each line with a "//". Do Ctrl-/ a second time reverses that. If the first line in the selection starts selecting at some other point than the first column on the line, I think (without actually checking) that it puts the "//" right before the first selected column.
The Eclipse default
Java editor can also do the block comments delimited by "/*" and "*/" and the rules are a bit stickier there since you cannot embed block comments. And again, I'm too lazy to check the details, but it's the same general idea.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.