That is incorrect. Java 6 and 8 can and do exist at the same time. Java 8 is installed only to the /Library/Internet Plug-Ins/ folder in the single package file JavaAppletPlugin.plugin. Java 6 is not. It's all over the place, including hidden folders, and none of it in the /Library/Internet Plug-Ins/ folder. So it's not even possible for Java 8 to overwrite 6.