Yes, of course you cannot edit them. They are part of libraries that Minecraft uses resp. part of Minecraft itself. You cannot just edit other people's code.
And why i saw people editing it?
in tutorials all this .class is .jar, and they edit by it.
i still don't understanding.
diesieben07, i cant understand it too, i dont know what is EntityJoinWorldEvent.
as i said, i'm beginner.