ArchiCAD 16 uses Java 6 for installing, patching and uninstalling ArchiCAD.
ArchiCAD will prompt to download Java 6 if it is not present during installation (see the attached screenshot). If you have a later version of Java (for instance Java 7) you can uninstall Java 6, but you will need to re-install it for hotfixes and uninstalling ArchiCAD.
Leaving two versions of Java on your system will not affect the operation of ArchiCAD.
Katalin Takacs